Wet type method of rendering dioxins innoxious

ABSTRACT

A process for the wet processing of a dioxin-containing material, wherein the material is contacted with an aqueous solution, acidified with hydrochloric acid and containing a catalyst dissolved therein, at a temperature lower than 100° C. to decompose the dioxins into harmless substances with a decomposition rate of at least 60%.

This application is a 371 of PCT/JP98/05978 Dec. 25, 1998.

TECHNICAL FIELD

The present invention relates to a process for the wet decomposition ofdioxins into harmless substances and to a process for the wet processingof a fly ash-containing gas discharged from a combustion furnace into aharmless substance.

BACKGROUND ART

Dioxins, the typical example of which is2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), are extremelyharmful to human bodies and the discharge thereof to an atmosphere isstrongly prohibited. Ministry of Health and Welfare of Japan published“Guideline for Prevention of Generation of Dioxins in Waste Treatment”in January, 1997, in which the concentration of dioxins in flue gasesdischarged from any newly constructed furnace is instructed to besuppressed to 0.1 ng-TEQ/Nm³ or less. Environment Agency of Japandesignates dioxins as being harmful substances by an amendment of theAtmospheric Pollution Prevention Law of December, 1997 and defines aregulating value for dioxins generated by combustion of industrialwastes as well as municipal solid wastes.

Various methods have been hitherto proposed for decomposing dioxins intoharmless substances. Examples of these methods include a combustionmethod, a melting method, a thermal decomposition method, aphoto-decomposition method, an ozone decomposition method, an oxidationdecomposition method using hydrogen peroxide, a hydrothermaldecomposition method and an alkali decomposition method.

The conventional methods, however, involve problems, because theyencounter great difficulties in practicing and because they are notsatisfactory from the standpoint of economy.

JP-A-H10-146574 proposes a method of decomposing dioxins, wherein adioxin-containing fly ash is mixed with an oxidative acid such assulfuric acid to form a slurry which is then heated at a temperaturehigher than 100° C.

With the above method, dioxins are efficiently converted into harmlesssubstances. However, the method has problems that energy consumption islarge and high apparatus costs are required, because the treatment isperformed at a temperature of at least 100° C. which is above theboiling point of water (at atmospheric pressure, and so forth),preferably at least 200° C., while evaporating water.

It is the prime object of the present invention to provide a process forthe wet decomposition of dioxins into harmless substances, which processis performed at a temperature lower than the boiling point of water andwhich can convert dioxins into harmless substances at a low cost.

Another object of the present invention is to provide a process for thewet processing of a flue gas from a combustion furnace containingdioxin-containing fly ash, which process is performed at a temperaturelower than the boiling point of water and which can convert dioxins intoharmless substances at a low cost.

The present inventors have made an intensive study for accomplishing theabove objects and unexpectedly found that dioxins can be converted intoharmless substances when contacted at a temperature lower than 100° C.with an aqueous solution acidified with hydrochloric acid and containinga catalyst dissolved therein and have completed the present invention.

In accordance with the present invention, there is provided a processfor the wet decomposition of dioxins into harmless substances,characterized in that the dioxins are contacted with an aqueoussolution, acidified with hydrochloric acid and containing a catalystdissolved therein, at a temperature lower than 100° C. to decompose thedioxins into harmless substances with a decomposition rate of at least60%.

The present invention also provides a process for the wet processing ofa flue gas from a combustion furnace containing a dioxin-containing flyash into a harmless substance, characterized in that said flue gas iscontacted with an aqueous solution, acidified with hydrochloric acid andcontaining a catalyst dissolved therein, at a temperature lower than100° C. to cause the fly ash contained in said flue gas to migrate intosaid aqueous solution and to decompose the dioxins deposited on the flyash into a harmless substance with a decomposition rate of at least 60%.

The present invention further provides a process for the wet processingof a flue gas generated from a combustion furnace, having a temperaturehigher than 100° C. and containing a dioxin-containing fly ash into aharmless substance, characterized in that said process comprises (i) acooling step of bringing the flue gas with gas-liquid contact with acooling liquid to reduce the temperature of the flue gas to below 100°C., (ii) a gas-liquid contacting step of bringing said flue gas obtainedin said cooling step into gas-liquid contact with an aqueous solutionacidified with hydrochloric acid, and (iii) a dioxin decomposing step ofsubjecting the fly ash-containing cooling liquid “A” obtained in saidcooling step and the fly ash-containing aqueous solution “B” obtained insaid contacting step, separately or jointly, to treatment conditionsincluding a chlorine ion concentration of at least 10 mmol/liter, acopper ion concentration of at least 20 mg/liter and a treatmenttemperature of lower than 100° C. to decompose the dioxins contained inthe fly ash into harmless substances.

The present invention further provides a process for the wet processingof a flue gas generated from a combustion furnace and containing adioxin-containing fly ash into a harmless substance, characterized inthat said process comprises (i) a gas-liquid contacting step of bringingsaid flue gas cooled to a temperature lower than 100° C. into gas-liquidcontact with an aqueous solution acidified with hydrochloric acid, (ii)a fly ash concentrating step of increasing a fly ash content of theaqueous solution obtained in said gas-liquid contacting step andcontaining fly ash, and (iii) a dioxin decomposing step of maintainingsaid aqueous solution, obtained in said fly ash concentrating step andcontaining an increased amount of the fly ash, at a temperature lowerthan 100° C. in the presence of a catalyst in a dissolved state, therebydecomposing the dioxins contained in the fly ash into harmlesssubstances.

The present invention further provides a process for the wet processingof a flue gas generated from a combustion furnace and containing adioxin-containing fly ash into a harmless substance, characterized inthat said process comprises (i) a first gas-liquid contacting step ofbringing said flue gas with a first treating liquid, (ii) a secondgas-liquid contacting step of bringing the treated flue gas obtained insaid first gas-liquid contacting step with a second treating liquid, and(iii) a dioxin decomposing step of contacting fly ash “A” captured bysaid first treating liquid in said first gas-liquid contacting step andfly ash “B” captured by said second treating liquid in said secondgas-liquid contacting step, separately or jointly, into contact with anaqueous solution, acidified with hydrochloric acid and containing acatalyst dissolved therein, to decompose the dioxins contained in thefly ash into harmless substances.

The term “dioxins” used in the present specification is intended torefer to 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) and similarcompounds thereof and to include polychlorodibenzo-p-dioxins (PCDDs)having 1-8 chlorine atoms in the dibenzo-p-dioxin structure andpolychlorodibenzofurans (PCDFs) having 1-8 chlorine atoms in thedibenzofuran nucleus.

As described above, dioxins include various chlorinated compounds. Sincethe toxicity of dioxins varies with the kind thereof, it is necessary toestablish a standard based on which toxicity of individual dioxins canbe evaluated in order to evaluate a mixture of dioxins as a whole. Forthis reason, a factor (toxicity equivalent factor (TEF)) for calculatingan amount of a dioxin in terms of an amount of 2,3,7,8-TCDD providingthe same toxicity as that of the dioxin has been determined on the basisof short time toxicity evaluation of respective dioxins. By multiplyingamounts of respective dioxins by the toxicity equivalent factor,toxicity equivalent quantities (TEQ) thereof can be obtained. Thetoxicity equivalent quantity is used for indicating a discharge amountand a concentration of dioxins.

The process for the treatment of dioxins into harmless substancesaccording to the present invention is characterized in that the dioxinsare contacted with an aqueous solution acidified with hydrochloric acidand containing a catalyst dissolved therein (hereinafter referred tosimply as reaction treatment agent or aqueous solution). The treatmenttemperature is lower than the boiling point of water (100° C.),preferably 80° C. or less. The lower limit is about 30° C.

The aqueous solution which is used as a reaction treating agent in thepresent invention has a Cl⁻ ion concentration of at least 10 mmol(millimol), preferably at least 100 mmol, per liter of the aqueoussolution. The upper limit is about 3,000 mmol. The pH of the aqueoussolution is 7 or less, preferably 6 or less, with a lower limit beinggenerally about 2. The aqueous solution can contain other inorganicacids such as sulfuric acid. In this case, it is advisable that themolar ratio [Cl⁻]/[SO₄ ²⁻] of the Cl⁻ to the SO₄ ²⁻ ion be 5 or more,preferably 20 or more. An upper limit is not specifically defined. As amethod of contacting dioxins with the aqueous solution, there may bementioned a method in which dioxins or dioxin-containing solids arestirred in the aqueous solution, a method in which the aqueous solutionis sprayed for contacting with dioxins or dioxin-containing solids, anda method in which contact is performed in a packed column or a platecolumn.

The term “aqueous solution acidified with hydrochloric acid” used in thepresent specification is intended to mean an aqueous acidic solutioncontaining chlorine ion. Examples of the acids used for maintaining theacidity may include hydrochloric acid, sulfuric acid and nitric acid.The use of hydrochloric acid is preferred.

The term “decomposition of dioxins” used herein is intended to refer toconversion of dioxins into detoxified compounds.

The aqueous solution acidified with hydrochloric acid used as thereaction treatment agent in the present invention contains a catalystwhich serves to promote the decomposition of dioxins. The inventors'studies have revealed that the contact of dioxins with the aqueoussolution can convert dioxins into harmless substances. When, however,the aqueous solution does not contain the catalyst, it takes a long timeto convert dioxins into harmless substances. Thus, from the industrialand commercial point of view, it is important that the catalyst shouldbe present in the aqueous solution. As the catalyst, a metal ion isused. The metal of such a metal ion may be, for example, iron,manganese, copper, nickel, cobalt, molybdenum, chromium, vanadium,tungsten, silver and tin. The above metal ions may be used by themselvesor as a mixture of two or more thereof. The metal ion may be an ordinarymetal ion or a complex ion. The use of a copper ion or an iron ion isfound to be preferred as a result of the inventors' studies. The amountof the metal ion contained in the aqueous solution is not specificallylimited. In the case of copper ion, the amount is 20-10,000 mg/liter,preferably 100-5,000 mg/liter, in terms of elemental copper. No increaseof the effect of the addition is expected even when the amount exceeds10,000 mg/liter. In the case of other metal ions, the amount thereof issimilar to that of the copper ion.

The catalyst used in the present invention is generally in the form of ametal oxide or a metal salt, such as a chloride, an oxide, a carbonateor a sulfate. The aqueous solution acidified with hydrochloric acid andused as the reaction treatment agent for rendering dioxins harmlesscontains such a metal oxide or metal salt in a dissolved state. In thiscase, however, the catalyst can contain an undissolved matter which isgenerally in the course of being converted into a dissolved state. Acatalyst containing such an undissolved matter in the course of beingconverted into a dissolved state can be effectively used.

As the catalyst used in the present invention, metal componentscontained in a combustion ashes such as a fly ash or a bottom ash may beutilized. Combustion ashes often contain metal components which canfunction as the above-described catalyst. By stirring such a combustionash in an aqueous solution acidified with hydrochloric acid, the metalcomponents contained therein can be utilized as the catalyst. Namely,the metal contained in the combustion ash is dissolved in the aqueoussolution to form a metal ion when the combustion ash is contacted withthe aqueous solution acidified with hydrochloric acid. The thus obtainedaqueous solution acidified with hydrochloric acid and containing such ametal ion dissolved therein can be used as the reaction treatment agentfor the wet decomposition of dioxins in the present invention.

The term “catalyst” used in the present specification is intended toinclude those which have a function to rendering dioxins harmless whilebeing changed their valence. Thus, the above-described metal ion used asthe catalyst does not necessarily have the same valence. The metal ionmay be a mixture of a low valence metal ion and a high valence metalion, for example, a mixture of a monovalent copper ion and a divalentcopper ion. A metal ion whose valence increases or decreases during thereaction may also be used. Illustrative of the suitable reactiontreatment agent is an aqueous solution acidified with hydrochloric acidand containing cuprous chloride and cupric chloride.

The aqueous solution used as the reaction treatment agent in the presentinvention may contain a substance (contact accelerating agent) capableof accelerating the contact of dioxins with the aqueous solution. Thecontact accelerating agent may include surfactants and alcohols. Thekinds of the surfactants are not specifically limited. Anionic,cationic, nonionic and amphoteric surfactants may be used. The amount ofthe surfactant added to the aqueous solution is 0.005-1% by weight,preferably 0.01-0.5% by weight. As the alcohol, a lower alcohol such asmethanol, ethanol or propanol may be suitably used. The amount of thealcohol is 0.5-10% by weight, preferably 1-10% by weight.

In the present invention, it is preferred that the reaction treatmentagent be irradiated with an ultrasonic wave for the purpose ofaccelerating the contact between the reaction treatment agent anddioxins. Ultrasonic waves customarily used for the formation ofemulsions may be suitably used for the purpose of the present invention.

If necessary, the reaction treatment agent can be contacted with oxygenor an oxygen-containing gas to enhance the concentration of oxygendissolved therein. As the method for contacting the aqueous solutionwith oxygen or an oxygen-containing gas, there may be mentioned a methodin which the oxygen or oxygen-containing gas is blown into the reactiontreatment agent, a method in which the oxygen or oxygen-containing gasis contacted with fine droplets of the reaction treatment agent and amethod in which the reaction treatment agent is brought intocounter-current contact with the oxygen or oxygen-containing gas in apacked column. As the oxygen-containing gas, there may be mentioned airand oxygen-enriched air.

In the process of converting dioxins into harmless substances accordingto the present invention, dioxins alone are rarely subjected to thetreatment. Rather, dioxins are generally treated in the state where theyare deposited on solids. Examples of the solids on which dioxins aredeposited include combustion ashes discharged from various kinds ofcombustion furnaces. The combustion ashes include fly ashes contained influe gases discharged from combustion furnaces and bottom ashesaccumulated on bottoms of furnaces. Such combustion ashes generallycontain unburnt carbon (or carbonaceous materials) within which dioxinsare present. Thus, it is very difficult to convert such dioxins intoharmless substances. Thus, when such combustion ashes are treated withthe reaction treatment agent into a harmless substance, it is preferredthat the contact accelerating agent for accelerating the contact betweenthe reaction treatment agent and the dioxins be used. Alternatively, forthe purpose of accelerating the contact between the reaction treatmentagent and the dioxins, it is effective to irradiate the reactiontreatment agent with an ultrasonic wave or pretreat the combustion ashesby, for example, pulverization or incineration to reduce the unburntcarbon content.

When a combustion ash is used as a raw material to be treated, thecombustion ash generally contains unburnt carbon or carbonaceousmaterials. Since the carbonaceous materials prevents the dioxinscontained therein from contacting with the aqueous solution, it isdifficult to convert the dioxins into harmless substances. Thisdifficulty increases as the amount of the carbonaceous materialsincreases. For this reason, it is preferred that the amount ofcarbonaceous materials contained in a combustion ash be previouslyreduced. According to the studies of the present inventors, it has beenfound to be desired that the amount of the carbonaceous materials be 2%by weight or less, preferably 1% by weight or less, more preferably 0.5%by weight or less. To achieve this purpose, it is preferred that thecombustion conditions in a combustion furnace for combusting wastes suchas refuses be controlled so that the amount of carbonaceous materialscontained in a combustion ash discharged therefrom is reduced. Further,in the case of a treatment of a fly ash or a bottom ash to which a largeamount of carbonaceous materials deposit, it is preferred that the ashbe combusted to reduce the amount of the carbonaceous materials beforetreating the ash into a harmless substance.

Solids having deposits of dioxins may be dioxin-polluted soils as wellas the above-described combustion ashes such as fly ashes and bottomashes.

According to the present invention, not only dioxins by themselves butalso such dioxins deposited on solids may be treated into harmlesssubstances. In this case, since the treatment temperature is lower thanthe boiling point of water, energy consumption is very small and theapparatus costs are low. The treatment time is about 1-100 hours.Concretely, however, the treatment time varies depending upon thetreatment conditions including the state in which dioxins to be treatedexist and the composition of the reaction treatment agent as well as thedecomposition rate of dioxins. Thus, it is difficult to unconditionallydetermine the treatment time. In the present invention, it is importantthat the dioxin decomposition rate should be at least 60%, preferably atleast 80%, more preferably at least 90%. In other words, by suitablyselecting the treatment conditions, inclusive of the composition of thereaction treatment agent and treatment time, the kind of the catalystcontained in the reaction treatment agent and the pretreatmentconditions for enhancing the reactivity of dioxins, etc., it is possibleto attain a dioxin decomposition rate of at least 60%, preferably atleast 80%, more preferably at least 90%. The process of decomposingdioxins with a decomposition rate of at least 60% by using a cheapreaction treatment agent such as an aqueous hydrochloric solution and alow treatment temperature which is greatly lower than the boiling pointof water has been first developed by the present inventors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a flow sheet for treating a dioxin-containingfly ash in accordance with the present invention (in FIG. 1, thereference numeral 10 designates a solid-liquid contacting device, 2 asolid-liquid separating device, 3 a metal separation device and 4designates a copper separating device);

FIG. 2 shows an example of a flow sheet for treating a flue gascontaining a fly ash discharged from a combustion furnace in accordancewith the present invention, (in FIG. 2, the reference numeral 40designates a combustion furnace (melting furnace));

FIG. 3 shows an example of a flow sheet for treating a flue gas from acombustion furnace in accordance with the present invention, wherein theflue gas contains hydrogen chloride and a fly ash containing a catalystmetal and wherein the treatment for converting dioxins contained in thefly ash into harmless substances is carried out in a reactor disposedseparately from a gas-liquid contacting device (in FIG. 3, the referencenumeral 70 designates a boiler, 71 a cooling device, 72 a gas-liquidcontacting device, 73 a heat exchanger, 74 a dust collector, 75 areactor, 76 a solid-liquid separating device and 77 designates a wastewater treatment device);

FIG. 4 shows another example of a flow sheet for treating a flyash-containing flue gas discharged from a combustion furnace accordingto the present invention (in FIG. 4, the reference numeral 1 designatesa combustion furnace, 2 a waste heat boiler, 3 a cooling tower, 4 agas-liquid contacting device, 5 a dust collector, 6 a reservoir, 7 athickener, 8 dioxin decomposition reactor and 9 designates asolid-liquid separating device);

FIG. 5 shows a variant of the flow sheet shown in FIG. 4 (in FIG. 5, thereference numerals similar to those of FIG. 4 designate the samecomponent parts and the reference numeral 10 designates an aqueoussolution concentrating device (multi-effect evaporator);

FIG. 6 shows a variant of the flow sheet shown in FIG. 4 (in FIG. 6,only a part of a total flow sheet that is related to a thickener 7 withthe other parts that are the same as in FIG. 4 being omitted);

FIG. 7 shows a flow sheet for increasing the concentration of fly ash inan aqueous solution in a thickener 7 (in FIG. 7, the reference numeral60 designates a fly ash slurry preparation vessel and the referencenumerals similar to those of FIG. 6 designate the same component parts);

FIG. 8 shows another example of a flow sheet for increasing theconcentration of fly ash in an aqueous solution in a thickener 7 (inFIG. 8, the reference numeral 60 designates a fly ash slurry preparationvessel and the reference numerals similar to those of FIG. 6 designatethe same component parts);

FIG. 9 is an example of a flow sheet for treating a fly ash-containingflue gas discharged from a combustion furnace by the process of thepresent invention using two Agas-liquid contacting steps (in FIG. 9, thereference numeral 1 designates a first gas-liquid contacting step, 2 asecond gas-liquid contacting step, 3 a dioxin decomposing step, 4 asolid-liquid separating step, 5 a waste water treating step and 6designates a fly ash treating step);

FIG. 10 is a flow sheet showing a detailed embodiment of the flow sheetof FIG. 9 (in FIG. 10, the reference numeral 31 designates a combustionfurnace, 32 a waste heat boiler, 33 a first gas-liquid contactingdevice, 34 a second gas-liquid contacting device, 35 a wet-type electricdust collector, 36 a dioxin decomposing reactor and 37 designates asolid-liquid separating device);

FIG. 11 is another example of a flow sheet showing a detailed embodimentof the flow sheet of FIG. 9 (in FIG. 11, the reference numeral 31designates a combustion furnace, 32 a waste heat boiler, 33 a firstgas-liquid contacting device, 34 a second gas-liquid contacting device,36 a dioxin decomposing reactor and 37 designates a solid-liquidseparating device);

FIG. 12 is a further example of a flow sheet showing a detailedembodiment of the flow sheet of FIG. 9 (in FIG. 12, the referencenumerals similar to those in FIG. 11 designate the same componentparts); and

FIG. 13 shows an experiment device used in a decomposition test fordioxins.

In the treatment of dioxin-containing fly ash according to the flowsheet shown in FIG. 1, the dioxin-containing fly ash is fed through aline 11 to the solid-liquid contacting device 10 maintained at atemperature lower than 100° C.

The solid-liquid contacting device 10, in which the dioxin-containingfly ash is contacted with an aqueous solution acidified withhydrochloric acid and containing a catalyst dissolved therein, isadapted to convert the dioxins into harmless substances. Thesolid-liquid contacting device 10 may have any desired structure as longas it permits a contact of a dioxin-containing fly ash with a liquid.

A make-up aqueous solution acidified with hydrochloric acid is fedthrough a line 13 to the solid-liquid contacting device 10 for beingcontacted with the dioxin-containing fly ash. If necessary, an aqueousalkaline solution is fed through a line 14 to the device 10. As theaqueous alkaline solution, there may be used a solution or slurryobtained by dissolving or dispersing an alkaline substance such assodium hydroxide, sodium carbonate, potassium hydroxide, calciumcarbonate or magnesium hydroxide in water.

The chlorine ion concentration of the aqueous solution used in thesolid-liquid contacting device 10 is at least 10 mmol/liter, preferablyat least 100 mmol/liter. The pH of the aqueous solution is 7 or less,preferably 6-2. A pH of less than 2 causes problems because corrosion ofthe device occurs significantly and because a difficulty is caused infiltering the aqueous solution after the dioxin decomposition treatment.It is desired that the aqueous solution contain a copper ion as thecatalyst. The copper ion is fed through a line 22. When the fly ashcontains copper, the copper is dissolved into the aqueous solution toform a copper ion. Therefore, the copper contained in the fly ash can beutilized as the copper ion. In this case, it is not necessary to feed acopper ion through the line 22. Copper obtained in the copper separationdevice 4 may be recycled to the device 10 through a line 21 for use asthe copper ion. The concentration of the copper ion in the aqueoussolution to be contacted with the dioxin-containing fly ash in thesolid-liquid contacting device 10 is 20-10,000 mg/liter, preferably100-5,000 mg/liter.

In the solid-liquid contacting device 10, the dioxin-containing fly ashis contacted with the aqueous solution to decompose the dioxins in thefly ash. In the present invention, at least 60%, preferably at least80%, more preferably at least 90% of the dioxins are decomposed. Toachieve this purpose, there may be employed a method in which asufficient residence time of the fly ash in the solid-liquid contactingdevice is ensured and in which the reaction time is controlled to beprolonged. In order to accelerate the decomposition of dioxins whilereducing the time required for the decomposition thereof (reactiontime), it is necessary to adjust the aqueous solution to have propertiessuited for the decomposition of the dioxins by incorporating a catalyst,especially copper ion, in the aqueous solution as described above.

Dioxin-containing fly ashes are discharged when, for example, cityrefuse is combusted. When the fly ash contains unburnt carbon, it issignificantly difficult to convert the dioxins into harmless substances.As described previously, in order to effectively decompose the dioxinsin such fly ashes, it is effective to reduce the amount of unburntcarbon deposited thereon to as low a level as possible. In the presentinvention, as described previously, it is desirable that the unburntcarbon content of the fly ash be 2% by weight or less, preferably 1% byweight or less, more preferably 0.5% by weight or less. To achieve thispurpose, it is necessary to completely combust a material to becombusted, such as refuse, in a combustion furnace in the presence ofsufficient oxygen and to reduce the amount of the carbonaceousmaterials. When a fly ash has a high unburnt carbon content, it ispreferred that the aqueous solution to be contacted with the fly ashcontain a contact-accelerating agent for accelerating the contactbetween the dioxins and the aqueous solution, as described previously.

The treated fly ash after being subjected to a solid-liquid contact inthe solid-liquid contacting device 10 is introduced through a line 15into the solid-liquid separation device 2 where it is subjected to asolid-liquid separation. The solid-liquid separation device 2 may haveany desired structure as long as it can separate a solid contained in aliquid. Examples of the separation devices include a filtering device, acentrifugal separation device and a sedimentation separation device.

In the solid-liquid separation device 2, solid substances such as flyash contained in the aqueous solution are separated. The aqueoussolution from which the solid substances have been separated isintroduced through a line 16 into the metal separation device 3. Themetal separation device 3 may have any desired structure as it canseparate heavy metal ions contained in the aqueous solution. Examples ofthe separation devices include a device adapted for sedimentating metalions as a precipitate and a device containing a metal ion absorbent(such as an ion-exchange resin or a chelating resin).

The metals separated in the metal separation device 3 are fed through aline 19 to the copper separation device 4 where a catalytic metal, suchas copper, contained therein is separated. The copper is fed, in theform of a water-soluble copper compound (such as copper chloride),through the line 21 to the solid-liquid contacting device 10. The copperseparation device 4 may have any desired structure as long as it canseparate copper from metals. Examples of such separation devices includea device adapted for selectively dissolving or precipitating a coppercompound among metal compounds and a device for selectively adsorbing acopper compound among metal compounds. In the present invention, it isnot necessary that the metal separation device 3 and the copperseparation device are provided separately. Only one device can be usedwhen the device has both of the functions of separating metals from anaqueous solution acidified with hydrochloric acid and of separatingcopper from metals.

The solution (waste solution) after the separation of metals in themetal separation device 3 is neutralized and then discharged through aline 18 and is discarded to a river, etc.

In the present invention, by maintaining the fly ash in contact with theaqueous solution for a predetermined period of time, the dioxinscontained in the fly ash can be almost completely decomposed intoharmless substances. In this case, for the purpose of enhancing thecontacting efficiency between the dioxins and the aqueous solution, itis desirably to add a contact-accelerating agent or to performultrasonic irradiation as described previously.

In the flow sheet shown in FIG. 2, the melting furnace 40 has an upperportion provided with a refuse feeding port 40 a to which shreddedrefuse is supplied by a dust supplier 42, such as a pusher using apiston or a pump, from a shredded refuse pit or hopper 41. To bottomportions of the furnace, there are connected an oxygen-enriched airfeeding pipe 43 and an auxiliary fuel supplying pipe 44. Theoxygen-enriched air is separated from air by a PSA separator and is usedfor maintaining the inside of the furnace at a high temperature bycontact with a char produced by thermal decomposition of the refuse.Disposed in the downstream side of the melting furnace 40 is a secondarycombustion chamber 45 for completely combusting a thermal decompositiongas obtained by the thermal decomposition of the refuse and being in areducing atmosphere. The secondary combustion chamber 45 is providedwith an air feeding pipe 51 and a water (such as dirty water from therefuse pit) feeding pipe 52. A waste heat recovering boiler is providedon an outlet side of the secondary combustion chamber 45. The heatrecovered in the boiler 46 is used for operating a turbine (not shown)connected to a generator, so that the above waste heat can be recoveredas an electric power.

Provided on a downstream side of the boiler 46 is a gas-liquidcontacting device 1 into which a flue gas containing a fly ash isintroduced after being cooled using a flue gas cooling water supply pipe32. The gas-liquid contacting device 1 has a water feeding pipe 13 and apH controlling aqueous alkaline solution feeding pipe 14. A solid-liquidseparation device 2 is connected to the gas-liquid contacting device 1through an aqueous solution transferring pipe 15. To the solid-liquidseparation device 2 is connected a metal separation device 3 to which acopper separation device 4 is connected.

The gas-liquid contacting device 1 has a flue gas outlet duct to which,through a mist separator (not shown), a heat exchanger 31 for heatingthe flue gas fed from the device 1 through a flue gas flow line 30, abag filter (dust collector) 33 for collecting a fly ash, etc. remainingin the flue gas and a heat exchanger 34 for heating the flue gas beforedischarge to the air for preventing the formation of white smoke aresuccessively connected in this order. The flue gas after the passagethrough the heat exchanger 34 is discharged to the air through a stack.A recycling line 36 is connected to the bag filter 33 for recycling thefly ash collected therein to the device 1.

In the combustion treatment of a waste material such as refuse and thetreatment of the flue gas from the combustion furnace according to theflow sheet shown in FIG. 2, the refuse is first fed to the meltingfurnace 40 from the refuse feeding port 40 a. The shredded refuse isthus thermally decomposed to form a thermal decomposition gas and acarbonaceous char having a high calorific value. The char is combustedwith the oxygen-enriched air and the auxiliary fuel supplied from thefeed lines 43 and 44, respectively, so that a bottom portion of themelting furnace is kept at a high temperature of about 1,650° C. As aresult, three, contiguous zones consisting of a bottom combustion andfusion zone, a middle thermal decomposition zone and a top drying zoneare formed within the refuse melting furnace 40. In the bottomcombustion and fusion zone, incombustible substances of the refuse areconverted into a non-polluting fused slug (a melt of metals and glass)which in turn is continuously discharged from the bottom of the furnace.At the same time, a high temperature gas generated by reactions in thebottom portion of the melting furnace (combustion furnace) 40 is movedupward to the thermal decomposition zone where the gas is used for thethermal decomposition of the feed of the refuse to form a thermaldecomposition gas in a reducing atmosphere. The refuse just fed tothrough the feed port 40 a is thus dried with the thermal decompositiongas.

The thermal decomposition gas is fed to the secondary combustion chamber45, to which air and dirty water are fed through the pipes 51 and 52,respectively, and is perfectly combusted. The waste heat is thermallyexchanged with steam in the boiler 46 and the heated steam is used fordriving the turbine and generator by which an electric power isrecovered. The flue gas from which the waste heat has been thusrecovered is contacted with water sprayed into a duct 11 through a fluegas cooling water feed pipe 32 and is thus adiabatically cooled to atemperature lower than 100° C. (generally about 65° C.). The cooled gasis then introduced into the gas-liquid contacting device 1.

In the gas-liquid contacting device 1, the flue gas is contacted withthe aqueous solution containing a catalytic metal ion (copper ion) tocapture the fly ash contained in the flue gas in the aqueous solutionand to extract metal components contained in the fly ash into theaqueous solution. Further, acidic gases, such as hydrogen chloride gas,contained in the flue gas are absorbed in the aqueous solution. In thegas-liquid contacting device 1, dioxins contained in the fly ash isalmost completely decomposed into harmless substances by retaining thefly ash captured in the aqueous solution for a predetermined period oftime. A portion of the liquid (slurry) in the device 1 is dischargedtherefrom and is fed to the solid-liquid separation device 2 through theline 15. A make-up water in an amount corresponding to the amount of thedischarged liquid is introduced into the device 1 through the line 13 sothat the amount of water in the system is maintained constant.

During the treatment of the flue gas in the device 1, the acidity of theaqueous solution may be lowered below a pH of 2 as time lapses, when theamount of the flue gas to be treated is large or when the concentrationof hydrogen chloride gas is high. In such a case, a quantity of analkaline water is fed through the line 14 to the aqueous solution sothat the pH thereof is maintained in a predetermined range.

The flue gas discharged from the gas-liquid contacting device 1generally has a dust concentration of about 0.1-0.3 g/Nm³. The flue gasis treated in a mist eliminator (not shown) for the removal of a misttherefrom and is then fed to the heat exchanger 31 where the flue gas isheated to a temperature higher by at least 20° C. than the watersaturated temperature so as to avoid occurrence of a trouble attributedto local condensation in the succeeding bag filter 33. The heated fluegas is then fed to the bag filter 33 where the fly ash remaining in theflue gas is removed. The flue gas thus converted into a harmlesssubstance is further heated with the heat exchanger 34 and thendischarged to the air from a stack.

The fly ash collected in the bag filter 33 is fed through the recyclingpipe 36 to the gas-liquid contacting device 1 where dioxins containedtherein are decomposed into harmless substances.

In the solid-liquid separation device 2, the fly ash in the aqueoussolution is separated. The separated fly ash which is high in safenessbecause dioxins have been decomposed into harmless substances isdischarged through a line 17 from the solid-liquid separation device 2.On the other hand, the aqueous solution from which the fly ash has beenremoved is fed through a line 16 to the metal separation device 3 whereheavy metal components dissolved therein are removed. The thus separatedmetals are fed through a line 19 to the copper separation device 4 toseparate copper. The separated copper is, if necessary after beingconverted into a water-soluble copper compound (such as copperchloride), recycled through a line 21 to the gas-liquid contactingdevice 1. The metals from which copper has been separated are dischargedthrough a line 20.

The aqueous solution from which metals have been separated in the metalseparation device 3 is discharged through a line 18.

When a flue gas from a combustion furnace contains hydrogen chloride orwhen a fly ash contained in the flue gas contains catalytic metals suchas copper, it is not necessary to add a catalyst or an aqueous acidicacid containing a chlorine ion from outside in order to treat the fluegas into a harmless substance according to the present invention. Insuch a case, the flue gas can be treated into a harmless substance byaddition of only an industrial water from outside.

Further, the above-described step of retaining the aqueous solutioncontaining the fly ash (dioxin decomposition reaction step) may becarried out not only in the gas-liquid contacting device but also in areactor to which the aqueous solution containing the fly ash is chargedafter being discharged from the gas-liquid contacting device. When theabove step is carried out in the reactor, it is easy to optimize thetreating conditions such as a temperature, a pH, a chlorine ionconcentration and a catalyst concentration. Therefore, it is preferableto perform the decomposition of dioxins in a fly ash into harmlesssubstances in a reactor disposed separately from the gas-liquidcontacting device.

FIG. 3 is an example of a flow sheet for treating dioxins contained in afly ash using a reactor disposed separately from a gas-liquid contactingdevice. The fly ash is contained in a hydrogen chloride-containing fluegas from a combustion furnace and contains a catalyst.

In the treatment of the flue gas according to the flow sheet shown inFIG. 3, the flue gas which has a high temperature (about 900° C.) andwhich is discharged from a combustion device is fed through a line 81 tothe boiler 70 to recover the heat thereof and to cool the flue gas toabout 250° C. The flue gas is then fed to the cooling device 71 andfurther cooled there. As a result, the temperature of the flue gas islowered to about 65° C.

The cooling device 71 has such a structure that a cooling water suppliedthrough a line 83 is sprayed to form fine droplets which are contactedwith the flue gas to adiabatically cool the flue gas. As a result of thecooling treatment of the flue gas in the cooling device 71, there areobtained a cooled flue gas and a cooling water which has caught aportion of the polluting substances, such as hydrogen chloride and thefly ash, contained in the flue gas upon contact therewith. The coolingwater after the contact of the flue gas has absorbed hydrogen chloridein the flue gas and is in the form of an aqueous hydrochloric acidsolution.

The flue gas and the aqueous hydrochloric acid solution in the coolingdevice 71 are introduced through lines 84 and 85, respectively, to thegas-liquid contacting device (exhaust gas washing device) 72 which maybe of a liquid dispersing type such as a spray tower or a packed columnor of a gas dispersing type such as a bubbling tower or a plate column.

As a result of the contacting treatment of the flue gas and the aqueoushydrochloric acid solution in the device 72, the polluting substances,such as acidic gasses and the fly ash, contained in the flue gas arealmost completely transferred to the aqueous hydrochloric acid solution.The thus cleaned flue gas is fed through a line 86 to the heat exchanger73, where the temperature thereof is increased by at least about 20° C.The heated flue gas is then fed through a line 87 to the dust collector74 to remove the fly ash remaining therein. The flue gas from which thefly ash has been removed is discharged through a line 88. The fly ashcollected in the dust collector 74 is separated and then transferredthrough a line 93 or 94 to the gas-liquid contacting device 72 or thereactor 75.

The aqueous solution obtained in the gas-liquid contacting device 72 andcontaining the fly ash which has been contacted with the flue gas isintroduced through a line 89 to the reactor 75, where dioxins containedin the fly ash are converted into harmless substances.

Namely, the ash-containing aqueous solution is stirred in the reactorfor a predetermined period of time during which the dioxins contained inthe fly ash are decomposed into harmless substances. In this case, theaqueous solution absorbs hydrogen chloride gas contained in the flue gasand dissolves the catalyst contained in the fly ash therein. Therefore,the aqueous solution shows a suitable function of decomposing dioxinsinto harmless substances. The reaction time, which depends upon theamount of carbonaceous materials contained in the fly ash, is difficultto be determined unconditionally. In the case where a dioxindecomposition rate of 80% is to be achieved, for example, the reactiontime is about 24-48 hours when the amount of carbonaceous materials inthe fly ash is 0.5% by weight or less, and about 50-60 hours when theamount of the carbonaceous materials is about 1.5% by weight. In thepresence of a contact-accelerating agent such as methanol, the reactiontime is about 20-30 hours, when the amount of the carbonaceous materialsis about 1.5% by weight. The reaction product obtained in the reactor 75is fed through a line 90 to the solid-liquid separation device 76 and issubjected to a solid-liquid separation treatment. The solids (fly ash)obtained are discharged through a line 92, while the aqueous solutionseparated is passed through a line 91 to a waste water treatment step 77including a metal separation treatment.

In the treatment of a flue gas from a combustion furnace into a harmlesssubstance according to the flow sheet shown in FIG. 4, the flue gascontaining dioxin-containing fly ash and generated in the combustionfurnace 1 is introduced into a waste heat boiler 2 through a line 11 torecover the heat thereof and is then fed to a cooling tower 3 through aline 12. The flue gas fed to the cooling tower 3 is contacted with finedroplets of a cooling liquid sprayed to an upper portion of the towerthrough lines 13 and 14. The flue gas is humidified and cooled by thiscontact to a temperature of about 60-75° C. which is a saturationtemperature determined by the amount of moisture contained in the fluegas before the cooling. The cooled flue gas is fed to a gas-liquidcontacting device 4 where it is contacted with an aqueous solutionacidified with hydrochloric acid. As a result of the contact, the flyash contained in the flue gas is captured by the aqueous solution and isremoved from the flue gas, whereas the flue gas is discharged through aline 23 from the device 4. The flue gas is then introduced into a heatexchanger 24 and is heated to such a temperature that no condensation ofwater vapors occurs in a bag filter 5 provided downstream thereof. Inthe bag filter 5, residual fly ash in the flue gas is removed. The fluegas which has been passed through the bag filter 5 is introduced througha line 25 to a heat exchanger 26 where it is heated for preventing theformation of white smoke. The flue gas is, if necessary, subjected to asuitable treatment and is then discharged to the air. The pH of theabove-described aqueous solution acidified with hydrochloric acid is 7or less, preferably 2-6.

As the gas-liquid contacting device 4, there may be used any device aslong as it has a function to capture fly ash contained in a flue gas ina liquid. For example, a vessel containing the aqueous solution may beused as such a device. The contact of the flue gas with the aqueoussolution in the vessel may be carried out by blowing the flue gasthrough a nozzle into the aqueous solution.

In the gas-liquid contacting device 4, the flue gas is contacted withthe aqueous solution acidified with hydrochloric acid and containingcatalyst metal ion (such as copper ion) dissolved therein, so that thefly ash contained in the flue gas is captured by the aqueous solutionwith the metal components in the fly ash being extracted with theaqueous solution. Additionally, acidic gas such as hydrogen chloridecontained in the flue gas is absorbed in the aqueous solution. Theliquid component (in the form of a slurry) in the device 4 is withdrawntherefrom and introduced into a thickener 7 through a line 28.

To the cooling tower 3, makeup cooling water is fed through lines 14 and15 so that the amount of liquid phase in the system is maintainedconstant. A portion of the cooling liquid used in the cooling tower 3 isfed through a line 16 to the gas-liquid contacting device 4.

As the treatment of the flue gas in the gas-liquid contacting device 4proceeds, the acidity of the aqueous solution may be reduced below 2when the amount of. the flue gas to be treated is large or when theconcentration of hydrogen chloride is high. In such a case, an alkalinewater is fed to the aqueous solution in such an amount as to maintainthe pH within a predetermined range according to a detection signal froma detector of a pH controlling device.

The thickener 7 serves to function as a concentrating device forincreasing the concentration of the fly ash in the fly ash-containingaqueous solution obtained in the gas-liquid contacting device 4. In thethickener 7, the fly ash in the aqueous solution falls by gravity sothat the fly ash concentration in a surface portion of the aqueoussolution is lowered while the fly ash concentration in a bottom portionis increased. The aqueous solution having an increased fly ashconcentration (fly ash-concentrated liquid) is fed through a line 29 anda pump to a dioxin decomposing reactor 8. The aqueous solution having alow fly ash concentration, on the other hand, is fed to a reservoir 6and is recycled through a pump and lines 21 and 20 to the cooling tower3 as a cooling liquid. A portion of the low fly ash aqueous solution inthe line 21 may be fed through a line 19 to the gas-liquid contactingdevice 4. In this case, a portion of the liquid in the line 19 may befed to the cooling tower 3 with the remainder portion being fed to thecontacting device 4.

The fly ash concentration rate in the thickener 7 is 2-60 times,preferably 3-50 times. It is preferred that the fly ash-concentratedliquid discharged from the thickener 7 have a fly ash concentration ofat least 1% by weight. The upper limit of the concentration is notspecifically limited but is generally about 30% by weight. The low flyash aqueous solution discharged from the thickener 7 and having areduced fly ash concentration has a fly ash concentration of not greaterthan 0.5% by weight, preferably not greater than 0.1% by weight.

By concentrating the fly ash-containing aqueous solution in thethickener 7 and by introducing the fly ash-concentrated liquid to thedioxin decomposing reactor 8, it is possible to make the reactor 8compact and to increase the efficiency thereof. In order to increase thedecomposition efficiency in decomposing dioxins in the reactor 8 intoharmless substances, it is necessary to increase the residence time(reaction time) of the fly ash in the device 8 as long as possible. Inthis case, when the fly ash concentration in the aqueous solution islow, it is necessary to increase the volume of the reactor incorrespondence to an increased residence time.

In the case of the present invention, since the fly ash-containingaqueous solution fed to the reactor 8 has a high fly ash concentration,the amount of fly ash per unit volume of the reactor is large so thatthe reactor may be made compact.

The low fly ash aqueous solution having a reduced fly ash concentrationand obtained in the thickener 7 is suited for use as a cooling liquidfor the cooling tower 3 and as a treating liquid for the gas-liquidcontacting device 4. Namely, because of the low fly ash content and offineness of the particle size of the fly ash, the aqueous solution doesnot cause troubles by friction and clogging in passage thereof throughpumps, pipes and nozzles and-can be handled in the same manner as theordinary industrial water.

In the dioxin decomposing reactor 8, dioxins contained in the fly ashare decomposed into a harmless state. Thus, the fly ash-containingaqueous solution is treated in the reactor with stirring for apredetermined period of time so that the dioxins in the fly ash aredecomposed into harmless substances. The aqueous solution contains acatalyst and exhibits good dioxin decomposing function. From thestandpoint of decomposition efficiency, a longer the residence time ofthe fly ash in the reactor 8 is preferable. Generally, however, thetreatment conditions are so adjusted that the residence time is in therange of 1-100 hours. The dioxin decomposing reactor 8 is operated toprovide a dioxin decomposition rate of generally at least 60%,preferably at least 80%, more preferably at least 90%.

The reaction product obtained in the reactor 8 is fed through a pump anda line 31 to a solid-liquid separating device 9 where it is subjected toa solid-liquid separating treatment. The solid phase (fly ash,etc.),obtained is discharged through a line 32, while the aqueoussolution separated is fed through a line 33 to a waste treatment stepincluding a metal separation treatment. A portion of the catalyst metalsuch as copper recovered by the metal separation treatment may berecycled to the dioxin decomposing reactor 8. In this case, the catalystconcentration in the dioxin decomposing reactor 8 is increased and isthus more effective.

The dioxin decomposing reactor 8 may be composed of a single reactionvessel or a plurality of reaction vessels. Since the reaction vessel isof a complete mixing vessel-type, a reactor having a good reactionefficiency may be obtained by connecting a plurality of reaction vesselsin series. When a plurality of reaction vessels are used, reactionconditions such as concentration of the catalyst, concentration ofchlorine ion and pH in respective reactors may be changed, so thatdioxins may be efficiently decomposed into harmless substances.

In the treatment of the flue gas into a harmless substance according tothe present invention, when the flue gas contains hydrogen chloride andwhen the fly ash contained in the flue gas contains catalyst metal suchas copper, it is not necessary to add a catalyst or a chlorineion-containing acidic aqueous solution from outside. Thus, it ispossible to treat the flue gas into a harmless substance by mereaddition of industrial water from outside.

In the present invention, the aqueous solution acidified withhydrochloric acid and containing a catalyst dissolved therein is used asa cooling liquid for the cooling step and as an aqueous solution for thegas-liquid contacting step. By this expedience, dioxins can bedecomposed during the cooling and gas-liquid contact.

Accordingly, the residence time of the fly ash-containing aqueoussolution in the dioxin decomposing reactor can be reduced. Thus, thereis obtainable a merit that the reactor can be made compact.

In the present invention, it is possible to use liquid cyclone in placeof the thickener. Similar to the thickener, the use of a liquid cyclonecan give both an aqueous solution having an increased fly ashconcentration and an aqueous solution having an decreased fly ashconcentration. The former is introduced into the dioxin decomposingreactor, while the latter is fed to the cooling tower and/or thegas-liquid contacting device.

The flow sheet shown in FIG. 5 differs from the flow sheet shown in FIG.4 in that, in the former, an aqueous solution, which is dischargedthrough a line 33 and from which fly ash has been separated in asolid-liquid separating device 9, is introduced to a concentratingdevice 10 and in that a part of the concentrated liquid is recycled to adioxin decomposing reactor 8.

The concentrating device 10 is composed of a multi-effect evaporator. Tothis device, heated steam is fed through a line 43, while an aqueoussolution from which fly ash has been separated is fed through a line 33.The heated steam and the aqueous solution thus introduced into thedevice 10 are indirectly contacted in a first evaporator “a”, so thatthe aqueous solution is indirectly heated with the heated steam. Thus,water is evaporated from the aqueous solution in an amount correspondingto the amount of the heat received, so that the aqueous solution isconcentrated. As a result of the indirect contact between the aqueoussolution and the heated steam, the steam is condensed to yield condensedwater which is discharged from a bottom of the evaporator “a” through aline 34.

The aqueous solution which has been subjected to the evaporation andconcentration treatment in the evaporator “a” is then introduced into anevaporator “b” where it is indirectly contacted with the steam generatedin the evaporator “a” and introduced to the evaporator “b”. Thus, theaqueous solution is indirectly heated with the steam, and water isevaporated from the aqueous solution in an amount corresponding to theamount of the heat received, so that the aqueous solution isconcentrated.

The condensed water produced as a result of the indirect contact betweenthe steam and the aqueous solution is discharged through a line 35.

The concentrated liquid and the evaporated steam in the evaporator “b”are introduced into an evaporator “c” in the same manner as above andare indirectly contacted with each other in the same manner as above.Condensed water produced in the evaporator “c” is discharged through aline 36. The evaporated steam from the evaporator “c” is passed througha line 37 to a cooler 38 where it is condensed. The condensed water isthen introduced into a cooling tower 3 through lines 39 and 42.

The concentrated liquid discharged through a line 40 from the evaporator“c” is introduced through a line 41 to a succeeding waste watertreatment device and, after being subjected to a suitable waste watertreatment there, is discarded. A portion of the concentrated liquid isrecycled through lines 40 and 44 to a dioxin decomposing reactor 8.Recycling of the concentrated liquid to the reactor 8 can increase thecatalyst concentration and chlorine ion concentration in the reactor,which contributes to both a reduction of the volume of the reactor andan improvement of the decomposition efficiency.

In the concentrating device 10, it is desirable that the aqueoussolution from the solid-liquid separating device 9 be concentrated tothe extent that no precipitation of chlorine compounds contained in theconcentrated liquid occurs.

In the embodiments shown FIGS. 4 and 5, the cooling tower using acooling liquid is employed to cool the flue gas from a combustionfurnace to a temperature lower than 100 ° C. Cooling of the flue gas maybe, however, performed by any other means such as by using a heatexchanger.

As described previously, the apparatus efficiency of the rector 8 ishigh when the fly ash concentration of the fly ash-containing aqueoussolution fed to the dioxin decomposing reactor 8 is high. For example,when the residence time of the fly ash-containing aqueous solution isthe same, doubling of the fly ash concentration in the aqueous solutioncan reduce the inside volume of the reactor to about ½. In order toincrease the fly ash concentration in the aqueous solution fed to thereactor 8, it is necessary to improve the concentrating efficiency ofthe thickener 7. Since the aqueous solution contains salts formed fromacidic gas in the flue gas and salts extracted from the fly ash, it isnecessary to remove these salts from the system. When the concentrationis performed to obtain a high fly ash concentration, the amount of theaqueous solution discharged is small. This results in an increase of thesalt concentration in the system. When the salt concentration becomesnear the saturation solubility, there occur such troubles that the saltsare apt to precipitate a locations of the system where the temperatureis cooled and that extraction of the salts from the fly ash is notcomplete. Therefore, when the concentration is carried out to obtain ahigh fly ash concentration, it is sometimes necessary to adjust theconcentration of the salts in the system.

In FIG. 6, only a part of a total flow sheet that is related to athickener 7 is shown, with the other parts of the flow sheet, which arethe same as the flow sheet of FIG. 4, being omitted.

According to the flow sheet shown in FIG. 6, exhaust water from thesystem is discharged not only through a line 29 but also through a line57. Therefore, it is possible to perform the concentration of the flyash while adjusting the concentration of salts in the system.

In the flow sheet shown in FIG. 7, a fly ash slurry prepared in a flyash slurry preparation vessel is used in place of the fly ash-containingaqueous solution discharged from the gas-liquid contacting device 4shown in FIG. 4. The flow sheet shown in FIG. 7 differs from the flowsheet for the treatment of the actual flue gas in that the formerincludes the fly ash slurry preparation vessel. However, the flow sheetof FIG. 7 is adopted because it is possible to easily carry out theconcentration experiment for the fly ash-containing aqueous solutionusing the thickener 7.

In FIG. 7, designated as 60 is the fly ash slurry preparation vessel.Reference numerals in FIG. 7 which are similar to those in FIG. 6designate the same component parts.

In the flow sheet shown in FIG. 7, fed to the fly ash slurry preparationvessel are a supernatant liquid in the thickener through a line 21,industrial water through a line 61, hydrochloric acid and sulfuric acid(simulation of acidic gas removed from the flue gas) through a line 62and fly ash through a line 63. While stirring these components, a pHcontrolling agent is added through a line 64 to form a slurry.

In the flow sheet shown in FIG. 8, a fly ash slurry prepared in a flyash slurry preparation vessel is used in place of the fly ash-containingaqueous solution discharged from the gas-liquid contacting device 4shown in FIG. 4. The flow sheet shown in FIG. 8 differs from the flowsheet for the treatment of the actual flue gas in that the formerincludes the fly ash slurry preparation vessel. However, the flow sheetof FIG. 8 is employed because it is possible to easily carry out theconcentration experiment for the fly ash-containing aqueous solutionusing the thickener 7.

In FIG. 8, designated as 60 is the fly ash slurry preparation vessel andas 10 a concentrating device by evaporation. Reference numerals in FIG.8 which are similar to those in FIG. 7 designate the same componentparts.

In the flow sheet shown in FIG. 8, an aqueous solution, which isdischarged through a line 33 from a solid-liquid separating device 9 andfrom which fly ash has been removed, is fed to the concentrating device10 where it is heated, so that part of water thereof is evaporated toeffect concentration. The concentrated liquid is discharged through aline 40. A part thereof is recycled though a line 44 to a reactor 8,with the remainder portion being discharged through a line 41. Steamgenerated in the concentrating device 10 is cooled and condensed in acooler 38 and discharged through a line 39.

In treatment of a flue gas from a combustion furnace into a harmlesssubstance according to the flow sheet shown in FIG. 9, the flue gas isintroduced into a first gas-liquid contacting step 1 through a line 11where it is contacted with a first treating liquid to capture acidic gasand fly ash contained therein and to remove them therefrom.

The first treating liquid used in the first treatment step 1 is formainly capturing hydrogen chloride (HCl) and fly ash contained in theflue gas. As the first treating liquid, there may be used industrialwater, an acidic aqueous solution or an aqueous solution acidified withhydrochloric acid and containing a dioxin decomposing catalyst. When anacidic aqueous solution or an aqueous solution acidified withhydrochloric acid is used, the pH of the solution is preferably adjustedto a range of 2-4. When an acidic solution having a pH of 2-4 is used asthe first treating liquid, it is possible to effectively absorb HClcontained in the flue gas in the treating liquid and to remove same fromthe flue gas.

When the flue gas is contacted with the first treating liquid, thetemperature and the amount of the first treating liquid are generallysuch that the temperature of the treating liquid upon the gas-liquidcontact is lower than 100° C., preferably lower than 80° C., though theydepend upon the temperature and amount of the flue gas. When thetemperature of the flue gas is 100° C. or more, for example, in therange of 110-300° C., the first treating liquid should possess afunction as a cooling liquid for the flue gas, so that it is necessaryto use a relatively large amount of the treating liquid having a lowtemperature or to use a number of spray nozzles to improve thegas-liquid contact thereof with the flue gas.

Through the gas-liquid contacting step using the first treating liquid,30-95%, preferably 80-95%, of the fly ash in the flue gas is captured inthe treating liquid. In such separation of the fly ash, those particleshaving large particle sizes are preferentially captured by the firsttreating liquid. Further, almost all of HCl in the flue gas is absorbedin the first treating liquid.

As a device for carrying out the first gas-liquid contacting step 1, anydevice may be used as long as it has a structure capable of capturingfly ash contained in the flue gas in the treating liquid. A devicehaving a liquid spraying nozzle and such as structure in which finedroplets sprayed from the spray nozzle are contacted with the gas or adevice having a gas blowing nozzle and a structure in which the gas isblown into the liquid may be used.

The flue gas which has been subjected to the contacting treatment withthe first treating liquid in the first gas-liquid contacting step 1 isintroduced through a line 12 into a second gas-liquid contacting step 2,where acidic gas and fly ash remaining in the flue gas are removed. Thesecond gas-liquid contacting step is mainly for separating SO₂ and flyash remaining in the flue gas therefrom.

In a first method for carrying out the second step, the flue gas iscontacted with the second treating liquid to remove SO₂ and fly ashremaining in the flue gas therefrom. At the same time, the flue gas iscooled in the second gas-liquid contacting step to a temperature lowerthan that of the flue gas at an inlet of the second gas-liquidcontacting step to condense moisture contained in the flue gas. A mistof the condensed water and the fly ash remaining therein are removed bya mist separator provided at an upper part of the second gas-liquidcontacting step. In this case, water mist is formed with fine particlesof the fly ash, which have passed through the first gas-liquidcontacting step and which remain in the flue gas, serving as nuclei.Thus, the weight and size of the particles are increased, so that theyare easily captured in the mist separator. As the mist separator, theremay be used a demister in which separation is performed by collision ora wet-type electric dust collector. The use of the wet-type electricdust collector is preferred. As a washing liquid for the wet-typeelectric dust collector, industrial water or the second treating liquidmay be used. The washing liquid preferably has a temperature equal to orlower than the temperature of the flue gas passing through the wet-typeelectric dust collector. For cooling the flue gas to a temperature lowerthan that of the flue gas at an inlet of the second gas-liquidcontacting step, there may be employed a direct cooling method in whichthe second treating liquid is contacted with the flue gas after beingcooled to a temperature below than that of the flue gas entering thesecond gas-liquid contacting step, or a method in which a heat exchangeris disposed in the second gas-liquid contacting step for indirectlycooling the flue gas. As the second treating liquid, condensed waterfrom the flue gas may be used as such. Alternatively, an acidic aqueoussolution or an aqueous solution acidified with hydrochloric acid andcontaining a catalyst for the decomposition of dioxins may be also used.

It is preferred that the pH of the second treating liquid be adjusted inthe range of 4-6 for reasons of effective removal of SO₂ remaining inthe flue gas. Further, by contact with the second treating liquid, thefly ash remaining in the flue gas may be removed and, hence, incombination of the removal of the water mist, the fly ash can be almostcompletely removed.

In the second gas-liquid contacting step 2, the flue gas is cooled to atemperature which is lower than that of the flue gas at an inlet of thesecond gas-liquid contacting step 2 and which is sufficient to condensewater vapors. Generally, the cooling temperature is a temperature lowerby 5-40° C., preferably 5-30° C., than that of the flue gas at an inletof the second gas-liquid contacting step 2.

As a device for carrying out the second gas-liquid contacting step 2, adevice generally used for contacting a gas with a liquid, such as aspray tower or a packed column may be used. When the above-describedindirect cooling with a heat exchanger is carried out, such a heatexchanger may be disposed in a suitable position within the spray toweror the packed tower.

The wet-type electric dust collector may be disposed in an upper portionwithin the tower or at a position outside the tower.

In a second method, a packed column in which activated carbon is packedis used in the second gas-liquid contacting step, so that not only SO₂and fly ash remaining in the flue gas but also gaseous dioxins areadsorbed and removed. Further, by contacting an aqueous solutionacidified with hydrochloric acid and containing a catalyst dissolvedtherein as the second treating liquid, with the packed layer ofactivated carbon, dioxins adsorbed by the activated carbon aredecomposed. The SO₂ remaining in the flue gas is, after having beenadsorbed to the activated carbon, converted to sulfuric acid by thecatalytic oxidation thereof and is removed. The sulfuric acid thusformed absorbs water contained in the flue gas to form dilute sulfuricacid which is liberated from the activated carbon and flowed downthrough the packed layer of the activated carbon. Upon the contact withthe second treating liquid, the fly ash remaining in the flue gas isalso removed. The contacting temperature in the second gas-liquidcontacting step 2 may be the same as the temperature of the flue gas atan inlet of the second gas-liquid contacting step but is generally30-80° C., preferably 50-70° C. The pH of the second treating liquid inthe second gas-liquid treating step 2 is not required to be high, sinceSO₂ is removed by adsorption on the activated carbon and catalyticoxidation with the activated carbon. It is preferred that the pH beadjusted in the range of 2-6 which is effective for decomposition ofdioxins adsorbed on the activated carbon.

The activated carbon preferably has such a form that does not causesignificant pressure loss of the flue gas, such as granules orhoneycomb. Granular activated carbon preferably has an average particlesize of at least 2 mm. Activated carbon generally used for gas treatmentor water treatment may be suitably used. It is preferred that theactivated carbon be subjected to a hydrophobicity-imparting treatmentsuch as heat treatment, fluorinating treatment or supporting ofhydrophobic particles of, for example, polypropylene, vinyl chlorideresin or tetrafluoroethylene. The amount of the hydrophobic particlessupported is 1-15% by weight, preferably 3-10% by weight. Ashoneycomb-like activated carbon, there may be used a molded productobtained by molding a mixture of powder of activated carbon with adispersing agent of hydrophobic particles such as tetrafluoroethylenedispersion into a honeycomb-like shape. The amount of the hydrophobicparticles may be 3-20% by weight, preferably 5-15% by weight. When suchactivated carbon is used, dioxins are effectively adsorbed on thehydrophobic surfaces and are then removed and decomposed into harmlesssubstances upon contact with the aqueous solution acidified withhydrochloric acid and containing a catalyst.

The treated flue gas obtained in the second gas-liquid contacting stepis discharged through a line 13 and, if necessary, subjected to atreatment for the prevention of white smoke before being discharged tothe air.

When a method in which the flue gas is cooled in the second gas-liquidcontacting step is adopted, the thermal load required for customarilyconducted heat treatment of the flue gas for the prevention of whitesmoke can be reduced, because the water content in the treated flue gasis decreased.

It is preferred that the second treating liquid discharged from thesecond gas-liquid contacting step 2 (second treatment waste liquid) berecycled through a line 23 to the first gas-liquid contacting step 1 andused as part of the first treating liquid. A part of the recyclingliquid may be directly fed to the decomposition step 3.

The fly ash captured by the first treating liquid (first treatmentexhaust liquid) in the first gas-liquid contacting step and the fly ashcaptured by the second treating liquid (second treatment exhaust liquid)in the second gas-liquid contacting step are, separately or jointly,brought into contact with an aqueous solution acidified withhydrochloric acid and containing a catalyst dissolved therein (treatingagent) in the dioxin decomposition step 3. Examples of methods forperforming the above step include a method in which the treating liquidcontaining these fly ashes is added into the treating agent and themixture is stirred for a predetermined period of time, and a method inwhich the gas-liquid contacting treatment is carried out using anaqueous solution acidified with hydrochloric acid and containing acatalyst dissolved therein as the first treating liquid and, if desired,as the second treating liquid and in which the fly ash-containingtreating liquid after the gas-liquid contacting treatment is fed to thereactor used in the decomposition step 3 and stirred for a predeterminedperiod of time. In the aqueous solution acidified with hydrochloric acidand containing a catalyst dissolved therein used as at least the firsttreating liquid, a metal ion for the catalyst may be added from outside.Alternatively, a metal ion dissolved from the fly ash upon contact ofthe fly ash with the aqueous solution acidified with hydrochloric acidmay be utilized as the catalyst. The hydrogen chloride (HCl) requiredfor the formation of the aqueous solution acidified with hydrochloricacid may be added from outside or may be available from hydrogenchloride contained in the flue gas. When the flue gas contains bothhydrogen chloride and catalyst metal-containing fly ash, the aqueoussolution acidified with hydrochloric acid and containing a catalystdissolved therein may be prepared by contacting the flue gas withindustrial water in the first gas-liquid contacting step.

The dioxin decomposition step 3 is generally performed so that thedecomposition rate of dioxins deposited on the fly ash is at least 60%,preferably at least 80%, more preferably at least 90%.

A conventional stirring-type reactor may be used as the decompositionvessel for the decomposition step 3. The decomposition step may becarried out by one stage treatment step or by an in series multi-stagestep.

The decomposition treatment liquid obtained in the decomposition step 3is fed to a solid-liquid separating step 4 for the separation of fly ashcontained therein. The fly ash is introduced through a line 16 into afly ash treating step 6 and treated there.

As a device for carrying out the solid-liquid separating step 4 for theseparation of fly ash contained in the treatment liquid, there may beused, for example, a filtration device or a centrifugal separator. Theseparated fly ash in the form of a solid (cake-like) obtained in thesolid-liquid separating step 4 is dried in the fly ash treating step 6.The dried fly ash is discharged through a line 18 for recovery asstabilized fly ash.

The separated liquid (waste water) obtained in the solid-liquidseparating step 4 is discharged through a line 17 and is discarded afterbeing subjected to a desired treatment in the waste water treating step5, if necessary.

A portion of the separated liquid obtained in the solid-liquidseparating step 4 is recycled through a line 19 to the first and/orsecond gas-liquid separating steps 1 and 2 for use as at least part ofthe first and/or second treating liquids. Thus, the catalyst accumulatedin the first treatment liquid can be used in the second gas-liquidcontacting step. If necessary, the catalyst may be added to the secondtreating liquid.

A line 22 connected to the second gas-liquid separating step 2 is usedfor feeding a part of the second treating liquid to a wet-type dustcollector provided in the second gas-liquid contacting step 2.

When the flue gas from a combustion furnace is treated into a harmlesssubstance in the manner described above, the fly ash-containing treatingliquid obtained in the first gas-liquid contacting step may be fed tothe reaction step 3 after increasing the fly ash concentration in thetreating liquid in a concentrating step.

As a device for use in the concentrating step, there may be used, forexample, a thickener or cyclone. In the concentrating step, a slurrycontaining fly ash at a high concentration, generally 1-20% by weight,preferably 4-20% by weight, and a low concentration liquid having a flyash concentration of 0.5% by weight or less, preferably substantiallyzero %. As described previously, the slurry is fed to the decompositionstep 3, while the low concentration liquid is fed to the first and/orsecond gas-liquid contacting steps 1 and 2 for use as part of the firstand/or second treating liquids.

In the treatment of a flue gas from a combustion furnace into a harmlesssubstance according to the flow sheet shown in FIG. 10, the flue gascontaining dioxin-containing fly ash and generated in the combustionfurnace 31 is introduced into a waste heat boiler 32 through a line 41to recover the heat thereof and is then fed to a first gas-liquidcontacting device 33 through a line 42. The flue gas introduced into thefirst gas-liquid contacting device is contacted with fine liquiddroplets of a first treating liquid fed through a line 45 and sprayedwithin the device and is rapidly cooled. At the same time, part of thefly ash and a greater part of HCl contained in the flue gas are capturedby the first treating liquid and separated from the flue gas. The fluegas is cooled to 45-75° C., preferably 50-70° C. by the first gas-liquidcontacting treatment. The treated flue gas which has been cooled andfrom which part of the fly ash and a greater part of HCl have beenremoved is fed through a line 43 to a second gas-liquid contactingdevice 34 where it is contacted with a second treating liquid. As aresult of this contact, the fly ash and SO₂ remaining in the flue gasare captured by the second treating liquid and removed from the fluegas.

The first gas-liquid contacting device 33 shown in FIG. 10 has a liquidspray nozzle at an upper part of the tower. The first treating liquidcontained in the bottom of the tower is fed through a line 45 andsprayed from the nozzle for contact with the gas. The structure of thedevice 33 is not limited to the above. For example, a structure in whichthe gas is blown through a nozzle into the liquid contained in a tankfor performing the gas-liquid contact may be also adopted.

The second gas-liquid contacting device 34 shown in FIG. 10 has a packedlayer formed therein. A liquid spray nozzle 57 is provided above thepacked layer F. The packing material for the packed layer may be anyknown material such as TELLERETTE. Provided further above the packedlayer F is a wet-type electric dust collector 35. The collector 35 isnot required to be disposed above the packed layer F but may be providedindependently from the second gas-liquid contacting device 34.

In the first gas-liquid contacting device 33, an aqueous solutionacidified with hydrochloric acid and containing a dioxin decomposingcatalyst is contained in the bottom thereof as the first treatingliquid. The aqueous solution is fed through a pump and a line 45 andsprayed from a top of the device, so that the device is filled with fineliquid droplets of the aqueous solution. The flue gas (having atemperature of generally 150-300° C.) introduced from the waste heatboiler 32 through a line 32 is contacted with the fine liquid dropletsso that the fly ash and HCl contained in the flue gas are capturedthereby and removed from the flue gas. The liquid droplets capturing thefly ash, etc. flow down through the device and is combined with theliquid in the bottom of the device. In the first gas-liquid contactingtreatment, the pH of the first treating liquid used in the firstgas-liquid contacting device is adjusted to 2-4 so that HCl contained inthe flue gas can be efficiently captured and separated by the treatingliquid.

As a result of the first gas-liquid contacting treatment, 30-95%,preferably 80-95% of the fly ash contained in the flue gas is capturedand removed by the first treating liquid. Further, at least 90%,preferably at least 95% of HCl contained in the flue gas is captured andremoved by the first treating liquid.

A part of the exhaust water obtained in the solid-liquid separatingdevice 37 is recycled through a line 51 to the second gas-liquidcontacting device 34 as the second treating liquid. The recycled liquidis fed to a lower part of the device 34 and is combined with the liquidcontained in the bottom of the device. The liquid contained in thebottom of the device is fed to a cooler 55 through a line 52 and a pumpand is cooled to a temperature of 30-50° C. A portion of the cooledliquid is fed through a line 56 and sprayed through a liquid spraynozzle 57, with the remainder portion being fed through a line 58 andsprayed through a liquid spray nozzle 59.

In the second gas-liquid contacting step 34, the second treating liquidflows downward through the packed layer F and is contacted with theupwardly flowing flue gas, so that the fly ash and SO₂ remaining in theflue gas are captured by the downwardly flowing treating liquid. Thetreating liquid capturing the fly ash flows down through the packedlayer F and is combined with the liquid contained in the bottom of thedevice.

In an upper space above the packed layer F of the second gas-liquidcontacting device 34, the cooled second treating liquid is sprayed and,hence, the temperature of the upper space is lower than the temperatureof the flue gas introduced through the line 43 to the device 34.Therefore, the moisture contained in the flue gas present at a positionbelow the space is condensed to form a mist. Since the water mist isformed with fine particles of the remaining fly ash serving as nuclei.Thus, there is formed an easily separable mist having a large weight andsize.

The mist-containing flue gas from the second gas-liquid separatingdevice 34 is fed a wet-type electric dust collector 35, where thecondensed water mist and fly ash-containing condensed water mist areremoved. The thus separated mist is washed away from electrode surfaceswith the cooled second treating liquid (mist removing treating liquid)sprayed from the spray nozzle 59 and streaming downward through thedevice. The liquid containing the washed mist is then passed through thepacked layer and is combined with the liquid contained in the bottom ofthe second gas-liquid contacting device.

The flue gas which has passed through the wet-type electric dustcollector 35 is discharged through a line 44 to the air after beingsubjected to a treatment for the prevention of white smoke, if desired.

It is preferred that the second treating liquid have a pH of 4-6 forreasons of efficient capture and removal of SO₂ contained in the fluegas.

A part of the liquid contained in the bottom of the second gas-liquidcontacting device 34 is recycled through a line 52, a pump and a line 53to the first gas-liquid contacting device 33 for use as the firsttreating liquid.

A part of the fly ash-containing first treating liquid contained in thebottom of the first gas-liquid contacting device 33 is fed through aline 46 to the dioxin decomposing reactor 36, where it is stirred for apredetermined period of time for the decomposition and removal ofdioxins deposited on the fly ash.

The treated liquid obtained in the dioxin decomposing reactor 36 isintroduced through a line 47 and a pump into a solid-liquid separatingdevice 37, where the fly ash contained therein is separated. Theseparated fly ash is discharged through a line 48. The separated water(exhaust water), on the other hand, is discharged through a line 49 anda pump. A portion of the separated water is recycled through lines 51and 61 to the first and second gas-liquid contacting devices 33 and 34,with the remainder portion being discarded through a line 70 after beingsubjected to a suitable exhaust water treatment, if necessary.

The make-up treating liquid is introduced through line 60 to the line51, if necessary.

In the treatment of a flue gas from a combustion furnace into a harmlessstate according to the flow sheet shown in FIG. 11, the flue gascontaining dioxin-containing fly ash and generated in the combustionfurnace 31 is introduced into a waste heat boiler 32 through a line 41to recover the heat thereof and is then fed to a first gas-liquidcontacting device 33 through a line 42. The flue gas introduced into thefirst gas-liquid contacting device 33 is contacted with fine liquiddroplets of a first treating liquid fed through a line 45 and sprayedwithin the device and is rapidly cooled. At the same time, part of thefly ash and a greater part of HCl contained in the flue gas are capturedby the first treating liquid and separated from the flue gas. The fluegas is cooled to 45-75° C., preferably 50-70° C. by the first gas-liquidcontacting treatment. The treated flue gas which has been cooled andfrom which part of the fly ash and a greater part of HCl have beenremoved is fed through a line 43 to a second gas-liquid contactingdevice 34 where it is contacted with a second treating liquid. As aresult of this contact, the fly ash and SO₂ remaining in the flue gasare captured by the second treating liquid and removed from the fluegas.

The first gas-liquid contacting device 33 shown in FIG. 11 has a liquidspray nozzle at an upper part of the tower. The first treating liquidcontained in the bottom of the tower is fed through a line 45 andsprayed from the nozzle for contact with the gas. The structure of thedevice 33 is not limited to the above. For example, a structure in whichthe gas is blown through a nozzle into the liquid contained in a tankfor performing the gas-liquid contact may be also adopted.

The second gas-liquid contacting device 34 shown in FIG. 11 has a packedlayer formed therein. A liquid spray nozzle 57 is provided above thepacked layer F.

In the first gas-liquid contacting device 33, an aqueous solutionacidified with hydrochloric acid and containing a dioxin decomposingcatalyst is contained in the bottom thereof as the first treatingliquid. The aqueous solution is fed through a pump and a line 45 andsprayed from a top of the device, so that the device is filled with fineliquid droplets of the aqueous solution. The flue gas (having atemperature of generally 150-300° C.) introduced from the waste heatboiler 32 through a line 32 is contacted with the fine liquid dropletsso that the fly ash and HCl contained in the flue gas are capturedthereby and removed from the flue gas. The liquid droplets capturing thefly ash, etc. flow down through the device and is combined with theliquid in the bottom of the device. In the first gas-liquid contactingtreatment, the pH of the first treating liquid used in the firstgas-liquid contacting device is adjusted to 2-4 so that HCl contained inthe flue gas can be efficiently captured and separated by the treatingliquid.

As a result of the first gas-liquid contacting treatment, 30-95%,preferably 80-95% of the fly ash contained in the flue gas is capturedand removed by the first treating liquid. Further, at least 90%,preferably at least 95% of HCl contained in the flue gas is captured andremoved by the first treating liquid.

A part of the exhaust water obtained in the solid-liquid separatingdevice 37 is recycled through a line 51 to the second gas-liquidcontacting device 34 as the second treating liquid. The recycled liquidis fed to a lower part of the device 34 and is combined with the liquidcontained in the bottom of the device. The liquid contained in thebottom of the device is fed through a line 52, a pump and a line 54 andsprayed through a liquid spray nozzle 57.

In the second gas-liquid contacting step 34, the second treating liquidflows downward through the packed layer F and is contacted with theupwardly flowing flue gas, so that the fly ash remaining in the flue gasis captured by the downwardly flowing treating liquid. The treatingliquid capturing the fly ash flows down through the packed layer F andis combined with the liquid contained in the bottom of the device. TheSO₂ contained in the flue gas, on the other hand, is adsorbed onactivated carbon and is converted into sulfuric acid by the catalyticoxidation and removed. The sulfuric acid thus formed absorbs watercontained in the flue gas to form dilute sulfuric acid which isliberated from the activated carbon, flowed down through the packedlayer of the activated carbon and combined with the liquid contained inthe bottom of the device.

The flue gas which has passed through the second gas-liquid contactingdevice 34 is discharged through a line 44 to the air after beingsubjected to a treatment for the prevention of white smoke, if desired.

It is preferred that the second treating liquid have a pH of 2-6 forreasons of efficient capture and removal of SO₂ contained in the fluegas.

A part of the liquid contained in the bottom of the second gas-liquidcontacting device 34 is recycled through a line 52, a pump and a line 53to the first gas-liquid contacting device 33 for use as the firsttreating liquid.

A part of the fly ash-containing first treating liquid contained in thebottom of the first gas-liquid contacting device 33 is fed through lines45 and 46 to the dioxin decomposing reactor 36, where it is stirred fora predetermined period of time for the decomposition and removal ofdioxins deposited on the fly ash.

The treated liquid obtained in the dioxin decomposing reactor 36 isintroduced through a line 47 and a pump into a solid-liquid separatingdevice 37, where the fly ash contained therein is separated. Theseparated fly ash is discharged through a line 48. The separated water(exhaust water), on the other hand, is discharged through a line 49 anda pump. A portion of the separated water is recycled through a line 51to the second gas-liquid contacting device 34, with the remainderportion being discarded through a line 70 after being subjected to asuitable exhaust water treatment, if necessary. The make-up treatingliquid is introduced through line 60 to the line 51, if necessary.

In the treatment of a flue gas from a combustion furnace into a harmlessstate according to the flow sheet shown in FIG. 12, the flue gascontaining dioxin-containing fly ash and generated in the combustionfurnace 31 is introduced into a waste heat boiler 32 through a line 41to recover the heat thereof and is then fed to a first gas-liquidcontacting device 33 through a line 42. The flue gas introduced into thefirst gas-liquid contacting device 33 is contacted with fine liquiddroplets of a first treating liquid fed through a line 45 and sprayedwithin the device and is rapidly cooled. At the same time, part of thefly ash and a greater part of HCl contained in the flue gas are capturedby the first treating liquid and separated from the flue gas. The fluegas is cooled to 45-75° C., preferably 50-70° C. by the first gas-liquidcontacting treatment. The treated flue gas which has been cooled andfrom which part of the fly ash and a greater part of HCl have beenremoved is fed through a line 43 to a second gas-liquid contactingdevice 34 where it is contacted with a second treating liquid. As aresult of this contact, the fly ash and SO₂ remaining in the flue gasare captured by the second treating liquid and removed from the fluegas.

The first gas-liquid contacting device 33 shown in FIG. 12 has a liquidspray nozzle at an upper part of the tower. The first treating liquidcontained in the bottom of the tower is fed through a line 45 andsprayed from the nozzle for contact with the gas. The structure of thedevice 33 is not limited to the above. For example, a structure in whichthe gas is blown through a nozzle into the liquid contained in a tankfor performing the gas-liquid contact may be also adopted.

The second gas-liquid contacting device 34 shown in FIG. 12 has a packedlayer F formed therein. A liquid spray nozzle 57 is provided above thepacked layer F.

In the first gas-liquid contacting device 33, an aqueous solutionacidified with hydrochloric acid and containing a dioxin decomposingcatalyst is contained in the bottom thereof as the first treatingliquid. The aqueous solution is fed through a pump and a line 45 andsprayed from a top of the device, so that the device is filled with fineliquid droplets of the aqueous solution. The flue gas (having atemperature of generally 150-300° C.) introduced from the waste heatboiler 32 through a line 32 is contacted with the fine liquid dropletsso that the fly ash and HCl contained in the flue gas are capturedthereby and removed from the flue gas. The liquid droplets capturing thefly ash, etc. flow down through the device and is combined with theliquid in the bottom of the device. In the first gas-liquid contactingtreatment, the pH of the first treating liquid used in the firstgas-liquid contacting device is adjusted to 2-4 so that HCl contained inthe flue gas can be efficiently captured and separated by the treatingliquid.

As a result of the first gas-liquid contacting treatment, 30-95%,preferably 80-95% of the fly ash contained in the flue gas is capturedand removed by the first treating liquid. Further, at least 90%,preferably at least 95% of HCl contained in the flue gas is captured andremoved by the first treating liquid.

A low fly ash concentration aqueous solution obtained in a thickener 61is recycled through a reservoir 63 and lines 64 and 66 to the secondgas-liquid contacting device 34 as the second treating liquid. Therecycled liquid is fed to a lower part of the device 34 and is combinedwith the liquid contained in the bottom of the device. The liquidcontained in the bottom of the device is fed through a line 52, a pumpand a line 54 and sprayed through a liquid spray nozzle 57.

In the second gas-liquid contacting step 34, the second treating liquidflows downward through the packed layer F and is contacted with theupwardly flowing flue gas, so that the fly ash remaining in the flue gasis captured by the downwardly flowing treating liquid. The treatingliquid capturing the fly ash flows down through the packed layer F andis combined with the liquid contained in the bottom of the device. TheSO₂ contained in the flue gas is adsorbed on activated carbon and isconverted into sulfuric acid by the catalytic oxidation and removed. Thesulfuric acid thus formed absorbs water contained in the flue gas toform dilute sulfuric acid which is liberated from the activated carbon,flowed down through the packed layer of the activated carbon andcombined with the liquid contained in the bottom of the device.

The flue gas which has passed through the second gas-liquid contactingdevice 34 is discharged through a line 44 to the air after beingsubjected to a treatment for the prevention of white smoke, if desired.

It is preferred that the second treating liquid have a pH of 2-6 forreasons of efficient capture and removal of SO₂ contained in the fluegas.

A part of the liquid contained in the bottom of the second gas-liquidcontacting device 34 is recycled through a line 52, a pump and a line 53to the first gas-liquid contacting device 33 for use as the firsttreating liquid.

A part of the fly ash-containing first treating liquid contained in thebottom of the first gas-liquid contacting device 33 is fed through lines45 and 46 to a concentration device (thickener) 61.

The thickener 61 serves to function as a concentrating device forincreasing the concentration of the fly ash in the fly ash-containingaqueous solution obtained in the first gas-liquid contacting device 33.In the thickener 61, the fly ash in the aqueous solution falls by thegravity so that the fly ash concentration in a surface portion of theaqueous solution is lowered while the fly ash concentration in a bottomportion is increased. The aqueous solution having an increased fly ashconcentration (fly ash-concentrated liquid) is fed through a line 67 anda pump to a dioxin decomposing reactor 36.

The aqueous solution having a low fly ash concentration, on the otherhand, is fed to a reservoir 63 and is recycled through a pump and lines64 and 66 to the second gas-liquid contacting device 34 as the secondtreating liquid. A portion of the low fly ash aqueous solution in theline 64 may be fed through a line 65 to the first gas-liquid contactingdevice 33.

The fly ash concentration rate in the thickener 61 is 2-60 times,preferably 3-50 times. It is preferred that the fly ash-concentratedliquid discharged from the thickener 61 have a fly ash concentration ofat least 1% by weight. The upper limit of the concentration is notspecifically limited but is generally about 30% by weight. The low flyash aqueous solution discharged from the thickener 7 and having areduced fly ash concentration has a fly ash concentration of not greaterthan 0.5% by weight, preferably not greater than 0.1% by weight.

By concentrating the fly ash-containing aqueous solution in thethickener 61 and by introducing the fly ash-concentrated liquid to thedioxin decomposing reactor 36, it is possible to make the reactor 36compact and to increase the efficiency thereof. In order to increase thedecomposition efficiency in decomposing dioxins in the reactor 36 intoharmless substances, it is necessary to increase the residence time(reaction time) of the fly ash in the device 36 as long as possible. Inthis case, when the fly ash concentration in the aqueous solution islow, it is necessary to increase the volume of the reactor incorrespondence to an increased residence time.

In the case of the present invention, since the fly ash-containingaqueous solution fed to the reactor 36 has high fly ash concentration,the amount of fly ash per unit volume of the reactor is large so thatthe reactor may be made compact.

The low fly ash aqueous solution having a reduced fly ash concentrationand obtained in the thickener 61 is suited for use as a cooling liquidfor the first gas-liquid contacting device 33 and as the treating liquidfor the second gas-liquid contacting device 34. Namely, because of thelow fly ash content and of fineness of the particle size of the fly ash,the aqueous solution does not cause troubles by friction and clogging inpassage thereof through pumps, pipes and nozzles and can be handled inthe same manner as the ordinary industrial water.

In the dioxin decomposing reactor 36, dioxins contained in the fly ashare decomposed and removed.

The treated liquid obtained in the dioxin decomposing reactor 36 isintroduced through a line 47 into a solid-liquid separating device 37,where the fly ash contained therein is separated. The separated fly ashis discharged through a line 48. The separated water (exhaust water), onthe other hand, is discharged through a line 49 and discarded afterbeing subjected to a suitable exhaust water treatment, if necessary. Themake-up treating liquid is introduced through line 60 to the secondgas-liquid contacting device 34, if necessary.

The present invention will be further described in detail by examples.

EXAMPLE 1

Dioxins contained in a fly ash having the properties shown below andseparated from a flue gas generated from a combustion furnace weretreated into harmless substances in a manner shown below.

Properties of Fly Ash

(i) Content of carbonaceous materials: 0.1 wt. % (ii) Content of copper:0.35 wt. % (iii) Content of dioxins: 3.6 ng-TEQ/g

Treating Method

The fly ash (400 g) was added to 2 liters of purified water, to whichhydrochloric acid was added with stirring and heating. The mixture wasthen maintained at 65° C. and a pH of 3.5 for 48 hours. The aqueoussolution had a Cl concentration of 1,900 mmol/liter, a molar ratio[Cl]/[SO₄] of 113 and a Cu concentration of 500 mg/liter. After the 48hour-stirring, the thus formed slurry was filtered to collect thetreated fly ash and to analyze dioxins. A dioxin decomposition rate wascalculated according to the following formula and to reveal that thedecomposition rate was 92%.

R=[(a₀×C₀)−(a×c)]/a₀c₀×100  (1)

R: DXN (dioxins) decomposition rate (%)

a₀: Weight of untreated fly ash, dry basis (g)

a: Weight of treated fly ash, dry basis (g)

c₀: DXN concentration of untreated fly ash (ng-TEQ/g)

c: DXN concentration of treated fly ash (ng-TEQ/g)

EXAMPLE 2

An experiment was carried out in the same manner as that in Example 1except that the pH adjustment was performed with sulfuric acid. TheCl/SO₄ molar ratio was 19. The dioxin decomposition rate was 61%.

EXAMPLE 3

An experiment was carried out in the same manner as that in Example 1except that the reaction time was changed. The dioxin decomposition ratewas 65% at a reaction time of 20 hours and 92% at a reaction time of 48hours.

EXAMPLE 4

Dioxins contained in a fly ash having the properties shown below andseparated from a flue gas from a combustion furnace were treated into aharmless state in a manner shown below.

Properties of Fly Ash:

(i) Content of carbonaceous materials: 3.0 wt. % (ii) Content of copper:0.17 wt. % (iii) Content of dioxins: 17.4 ng-TEQ/g

Treating Method:

The fly ash (400 g) was added to 4 liters of purified water containing10 wt % of methanol as a contact-accelerating agent, to whichhydrochloric acid was added with stirring and heating. The mixture wasthen maintained at 65° C. and a pH of 3.5 for 48 hours. The aqueoussolution had a Cl concentration of 480 mmol/liter, a molar ratio[Cl]/[SO₄] of 29 and a Cu concentration of 100 mg/liter. After the 48hour-stirring, the thus formed slurry was filtered to collect thetreated fly ash and to analyze dioxins. A dioxin decomposition rate wascalculated to reveal that the decomposition rate was 70%.

EXAMPLE 5

An experiment was carried out in the same manner as that in Example 4except that the aqueous solution was irradiated with an ultrasonic wave.The dioxin decomposition rate was 83%. An ultrasonic wave treatingdevice (UP-50H manufactured by Kubota Shoji Inc.) was used forgenerating the ultrasonic wave.

EXAMPLE 6

An experiment was carried out in the same manner as that in Example 4except that 8 g of a sulfosuccinate-type anionic surfactant and 4 g ofan ether-type nonionic surfactant were substituted for the methanol. Thedioxin decomposition rate was 67%.

EXAMPLE 7

An experiment of decomposition of dioxins contained in a fly ash wascarried out using an apparatus shown in FIG. 13 in the manner and underthe conditions shown below.

Treatment Procedure and Conditions:

1. In a flask shown in FIG. 13, 2.0 liters of purified water containing100 mg/liter of each of Fe, Mn, Mo, Cu, Zn, Cr and V (each dissolved asa chloride) were charged, to which a small amount of hydrochloric acidwas added. The mixture was heated with stirring and maintained at a pHof 3.5 and a temperature of 65° C.

2. Then, 400 g of fly ash as used in Example 1 were added to the abovesolution. The mixture was then maintained at a pH of 3.5 by addition ofhydrochloric acid with stirring at 65° C. for 48 hours while bubblingair at 50 N liter/hour.

3. After the 48 hours stirring, the slurry was filtered to obtain atreating liquid and a treated fly ash.

4. The raw material fly ash, the treating liquid, the treated fly ashand an exhaust gas discharged overhead from a condenser mounted on theflask were analyzed for dioxins.

Results of Treatment:

(1) The weight change of the fly ash before and after the wet processingis as shown in Table 1 below. Thus, the weight of 400 g before thetreatment decreased to 150 g (37.5% of the weight before the treatment).The decrease is considered to be attributed to the dissolution ofsoluble salts such as NaCl.

TABLE 1 Weight change of fly ash before and after the wet processing(unit: g (dry base)) Before treatment After treatment Weight of fly ash400 150

(2) The concentrations of the dioxins in the fly ash before and afterthe above wet porcessing were as shown in Table 2. The concentrations inthe fly ash after the treatment were significantly lower as a whole thanthose before the treatment. The acronyms in Tables 2 and 3 mean asfollows:

T4CDDs: tetrachlorodibenzo-p-dioxins

p5CDDs. pentachlorodibenzo-p-dioxins

H6CDDs: hexachlorodibenzo-p-dioxins

H7CDDs: heptachlorodibenzo-p-dioxins

O8CDDs: octachlorodibenzo-p-dioxins

Total PCCDs: total polychlorodibenzo-p-dioxins

T4CDFs: tetrachlorodibenzofuran

P5CDFs: pentachlorodibenzofuran

H6CDFs: hexachlorodibenzofuran

H7CDFs: heptachlorodibenzofuran

O8CDFs: octachlorodibenzofuran

Total PCDFs: total polychlorodibenzofuran

Total: Total dioxins

TABLE 2 Concentrations of dioxins in fly ash before and after the wetprocessing (unit: ng-TEQ/g) Before treatment After treatment (rawmaterial (treated fly fly ash) ash) Dioxin T4CDDs 0.064 0.013 P5CDDs0.27 0.041 H6CDDS 0.35 0.098 H7CDDs 0.13 0.054 O8CDDs 0.033 0.019 Total0.85 0.23 PCDDs Dibenzo- T4CDFs 0.04 0.0089 furan P5CDFs 0.90 0.25H6CDFs 1.0 0.42 H7CDFs 0.22 0.15 O8CDFs 0.0082 0.0076 Total 2.2 0.83PCDFs Total 3.1 1.1

(3) The removal rate (decomposition rate) of dioxins is shown in Table3. The removal rate was calculated according to the formula (1) above,bearing mind that the weight of the fly ash after the treatment was37.5% of the original weight. Since the amount of dioxins contained inthe exhaust gas and the treating liquid was ignorable from thestandpoint of mass balance, the removal rate was calculated only fromthe analyzed values of dioxin concentrations in the fly ash.

TABLE 3 Removal Rate of Dioxins (unit: %) Removal Rate Dioxin T4CDDs 92P5CDDs 94 H6CDDs 89 H7CDDs 84 O8CDDs 78 Total PCDDs 90 Dibenzo- T4CDFs92 furan P5CDFs 90 H6CDFs 84 H7CDFs 75 O8CDFs 65 Total PCDFs 86 Total 87

EXAMPLE 8

Fly ash separated and recovered from a flue gas generated from acombustion furnace was treated into a harmless state according to theflow sheet shown in FIG. 7. The main operation conditions were as shownbelow with reference to FIG. 7.

Method of Experiment

To the fly ash slurry preparation vessel 60 shown in FIG. 7 were fed athickener supernatant through the line 21, industrial water through theline 61, hydrochloric acid and sulfuric acid (molar ratio: 7:1,simulation of acidic gas removed from flue gas) through the line 62 and50 g/h of fly ash through the line 63. While stirring the mixture,magnesium hydroxide was added through the line 64 to form a slurryhaving a pH of 3.5. The residence time of the liquid in the slurrypreparation vessel 60 was simulation of the residence times of thecooling tower 3 and the gas-liquid contacting device 4 shown in FIG. 4.

(1) Line 63

Amount of fly ash: 50 g/h

Concentration of dioxins in fly ash: 1.0 ng-TEQ/g

(2) Line 28

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 36,200 ppm-w (ppm-w: concentration on weightbasis)

Cu ion concentration: 120 ppm-w

Cl/SO₄ molar ratio: 100

PH: 3.5

Fly ash concentration: 0.5% by weight

Discharge rate: 4,900 g/h

(3) Line 27

Temperature: 65° C.

Fly ash concentration: 0.1% by weight

Overflowing rate: 3,500 g/h

(4) Line 29

Temperature: 65° C.

Fly ash concentration: 1.5% by weight

Concentration of dioxins in fly ash: 1.8 ng-TEQ/g

Discharge rate: 1,400 g/h

(5) Dioxin decomposition reactor 8

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 36,200 ppm-w

Cu ion concentration: 120 ppm-w

Cl/SO₄ molar ratio: 100

PH: 3.5

Fly ash concentration: 1.5% by weight

Residence time: 48 hours

(6) Line 31

Temperature: 65° C.

Fly ash concentration: 1.5% by weight

Concentration of dioxins in fly ash: 0.26 ng-TEQ/g

Dioxin decomposition rate: 89%

(7) Line 32

Amount of fly ash: 21 g/h

Concentration of dioxins in fly ash: 0.26 ng-TEQ/g

Dioxin decomposition rate: 89%

EXAMPLE 9

Fly ash separated and recovered from a flue gas from a combustionfurnace was treated into a harmless state according to the flow sheetshown in FIG. 8. The main operation conditions were as shown below withreference to FIG. 8.

Method of Experiment

To the fly ash slurry preparation vessel 60 shown in FIG. 8 were fed athickener supernatant through the line 21, industrial water through theline 61, hydrochloric acid and sulfuric acid (molar ratio: 7:1,simulation of acidic gas removed from flue gas) through the line 62 and50 g/h of fly ash through the line 63. While stirring the mixture,magnesium hydroxide was added through the line 64 to form a slurryhaving a pH of 3.5. The residence time of the liquid in the slurrypreparation vessel 60 was simulation of the residence times of thecooling tower 3 and the gas-liquid contacting device 4 shown in FIG. 5.The concentrating device 10 used an electric heater.

(1) Line 63

Amount of fly ash: 50 g/h

Concentration of dioxins in fly ash: 1.0 ng-TEQ/g

(2) Line 28

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 36,200 ppm-w (ppm-w: concentration on weightbasis)

Cu ion concentration: 120 ppm-w

Cl/SO₄ molar ratio: 100

PH: 3.5

Fly ash concentration: 0.5% by weight Discharge rate: 4,900 g/h

(3) Line 27

Temperature: 65° C.

Fly ash concentration: 0.1% by weight

Overflowing rate: 3,500 g/h

(4) Line 29

Temperature: 65° C.

Fly ash concentration: 1.5% by weight

Concentration of dioxins in fly ash: 1.8 ng-TEQ/g

Discharge rate: 1,400 g/h

(5) Dioxin decomposition reactor 8

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 60,000 ppm-w

Cu ion concentration: 200 ppm-w

Cl/SO₄ molar ratio: 160

PH: 3.5

Fly ash concentration: 1.1% by weight

Residence time: 35 hours

(6) Line 31

Temperature: 65° C.

Fly ash concentration: 1.1% by weight

Concentration of dioxins in fly ash: 0.21 ng-TEQ/g

Dioxin decomposition rate: 91%

Amount of liquid treated: 1,945 g/h

(7) Line 32

Amount of fly ash: 21 g/h

Concentration of dioxins in fly ash: 0.21 ng-TEQ/g

Dioxin decomposition rate: 91%

(8) Line 40

Properties of concentrated liquid:

Cl ion concentration: 120,000 ppm-w

Cu ion concentration: 400 ppm-w

Amount of concentrated liquid: 960 g/h

(9) Line 39

Amount of condensed water: 964 g/h

(10) Line 44

Amount of recycled liquid: 545 g/h

(11) Line 41

Amount of discharged liquid: 415 g/h

EXAMPLE 10

Fly ash was treated into a harmless state according to the flow sheetshown in FIG. 7. In this method, concentration with the thickener wascarried out so that the fly ash concentration became 4% by weight.However, since the size of the reactor for decomposition of dioxins wasreduced to about {fraction (1/2.5)} of the reactor used in Example 8,the residence time was the same as in Example 8.

(1) Line 63

Amount of fly ash: 50 g/h

Concentration of dioxins in fly ash: 1.0 ng-TEQ/g

(2) Line 28

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 99,000 ppm-w

(ppm-w: concentration on weight basis)

Cu ion concentration: 330 ppm-w

Cl/SO₄ molar ratio: 250

PH: 3.5

Fly ash concentration: 0.5% by weight

Discharge rate: 5,120 g/h

(3) Line 27

Temperature: 65° C.

Fly ash concentration: 0.1% by weight

Overflowing rate: 4,595 g/h

(4) Line 29

Temperature: 65° C.

Fly ash concentration: 4.0% by weight

Concentration of dioxins in fly ash: 1.7 ng-TEQ/g

Discharge rate: 525 g/h

(5) Dioxin decomposition reactor 8

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 99,000 ppm-w

Cu ion concentration: 330 ppm-w

Cl/SO₄ molar ratio: 250

PH: 3.5

Fly ash concentration: 4.0% by weight

Residence time: 48 hours

(6) Line 31

Temperature: 65° C.

Fly ash concentration: 4.0% by weight

Concentration of dioxins in fly ash: 0.13 ng-TEQ/g

Dioxin decomposition rate: 95%

(7) Line 32

Amount of fly ash: 21 g/h

Concentration of dioxins in fly ash: 0.13 ng-TEQ/g

Dioxin decomposition rate: 95%

EXAMPLE 11

Fly ash was treated into a harmless state according to the flow sheetshown in FIG. 6. The main operation conditions were as shown below withreference to FIG. 6. Since the preparation of fly ash slurry was thesame as that in Example 8, description is omitted here. In this method,the concentration with the thickener was carried out so that the fly ashconcentration became 10% by weight. However, since the size of thereactor for decomposition of dioxins was reduced to about ⅙ of thereactor used in Example 8, the residence time was the same as in Example8.

(1) Line 28

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 50,000 ppm-w

Cu ion concentration: 170 ppm-w

Cl/SO₄ molar ratio: 140

PH: 3.5

Fly ash concentration: 0.5% by weight

Discharge rate: 5,195 g/h

(2) Line 27

Temperature: 65° C.

Fly ash concentration: 0.1% by weight

Overflowing rate: 4,985 g/h

(3) Line 29

Temperature: 65° C.

Fly ash concentration: 10% by weight

Concentration of dioxins in fly ash: 1.8 ng-TEQ/g

Discharge rate: 210 g/h

(4) Dioxin decomposition reactor 8

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 50,000 ppm-w

Cu ion concentration: 170 ppm-w

Cl/SO₄ molar ratio: 140

PH: 3.5

Fly ash concentration: 10% by weight

Residence time: 48 hours

(5) Line 31

Temperature: 65° C.

Fly ash concentration: 10% by weight

Concentration of dioxins in fly ash: 0.22 ng-TEQ/g

Dioxin decomposition rate: 91%

(6) Line 32

Amount of fly ash: 21 g/h

Concentration of dioxins in fly ash: 0.22 ng-TEQ/g

Dioxin decomposition rate: 91%

(7) Line 57

Properties of discharged liquid:

Cl ion concentration: 50,000 ppm-w

Cu ion concentration: 170 ppm-w

Amount of discharged liquid: 810 g/h

(8) Line 58

Temperature: 65° C.

Fly ash concentration: 0.1% by weight

COMPARATIVE EXAMPLE

In Example 8, the slurry having a fly ash concentration of 0.5% byweight was fed to the dioxin decomposing reactor 8 without using thethickener. The results are shown below.

(1) Liquid feed to dioxin decomposing reactor (Line 29)

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 11,900 ppm-w

Cu ion concentration: 40 ppm-w

Cl/SO₄ molar ratio: 33

PH: 3.5

Fly ash concentration: 0.5% by weight

Concentration of dioxins in fly ash: 2.1 ng-TEQ/g

Liquid feed rate: 4,200 g/h

(2) Dioxin decomposition reactor 8

Temperature: 65° C.

Properties of discharged liquid:

Cl ion concentration: 11,900 ppm-w

Cu ion concentration: 40 ppm-w

Cl/SO₄ molar ratio: 33

PH: 3.5

Fly ash concentration: 0.5% by weight Residence time: 16 hours

(3) Liquid in outlet of dioxin decomposing reactor (Line 31)

Temperature: 65° C.

Fly ash concentration: 0.5% by weight

Concentration of dioxins in fly ash: 1.4 ng-TEQ/g

Dioxin decomposition rate: 41%

(4) Line 32

Amount of fly ash: 21 g/h

Concentration of dioxins in fly ash: 1.4 ng-TEQ/g

Dioxin decomposition rate: 41%

EXAMPLE 12

Flue gas from a combustion furnace was treated into a harmless stateaccording to the flow sheet shown in FIG. 12. Within the secondgas-liquid contacting device, a packed layer containing activated carbonhoneycomb prepared from a mixture of coal-type activated carbon withtetrafluorocarbon dispersion (tetrafluoroethylene content* 10% byweight) was disposed. The main operation conditions were as shown belowwith reference to FIG. 12. The data in this example were based onresults obtained in respective small-scale experiments.

(1) Line 42

Temperature: 250° C.

Amount of gas: 15,000 Nm³/h (dry basis)

Concentration of hydrochloric acid in gas: 580 ppm

Concentration of SO₂ in gas: 100 ppm

Amount of fly ash in gas: 19,000 g/h

Concentration of fly ash in gas: 1.27 g/Nm³

Concentration of dioxins in fly ash: 1.0 ng-TEQ/g

Total dioxin concentration in gas: 1.33 ng-TEQ/ Nm³

(2) First gas liquid contacting device 33

Cooling temperature: 70° C.

(3) Line 43

Temperature: 70° C.

Concentration of hydrochloric acid in gas: 10 ppm

Concentration of SO₂ in gas: 80 ppm

Amount of fly ash in gas: 1,900 g/h

(4) Line 44

Temperature: 70° C.

Concentration of hydrochloric acid in gas: 0 ppm

Concentration of SO₂ in gas: 7 ppm

Amount of fly ash in gas: 200 g/h

Total dioxin concentration in gas: 0.023 ng-TEQ/ Nm³

Total dioxin removing rate in gas: 98.3%

(5) Line 54

Temperature of second treating liquid: 70° C.

(6) Line 46

Temperature: 70° C.

Fly ash concentration: 0.5% by weight

(7) Line 62

Temperature: 70° C.

Fly ash concentration: 0.1% by weight

(8) Line 67

Temperature: 70° C.

Properties of discharged liquid:

Cl ion concentration: 99,000 ppm-w (ppm-w: concentration on weightbasis)

Cu ion concentration: 330 ppm-w

Cl/SO₄ molar ratio: 250

PH: 3.5

Fly ash concentration: 4% by weight

Concentration of dioxins in fly ash: 1.8 ng-TEQ/g

(9) Dioxin decomposing reactor 36

Temperature: 70° C.

Properties of discharged liquid:

Cl ion concentration: 99,000 ppm-w

Cu ion concentration: 330 ppm-w

C1/SO₄ molar ratio: 250

PH: 3.5

Fly ash concentration: 4% by weight

Residence time: 48 hours

(10) Line 47

Temperature: 70° C.

Fly ash concentration: 4%,by weight

Concentration of dioxins in fly ash: 0.14 ng-TEQ/g

(11) Line 48

Amount of fly ash: 8.0 kg/h

Concentration of dioxins in fly ash: 0.14 ng-TEQ/g

Dioxin decomposing rate in fly ash: 94%

(12) Line 49

Concentration of dioxins in waste water: 0.001 ng-TEQ/kg

Decomposing rate of total dioxins removed: 94%

According to the present invention, dioxins in fly ash contained in aflue gas from a combustion furnace can be decomposed into a harmlessstate with a high efficiency and at low costs.

What is claimed is:
 1. A process for the wet decomposition of dioxinsinto harmless substances, characterized in that the dioxins arecontacted with an aqueous solution, acidified with hydrochloric acid andcontaining a catalyst dissolved therein, at a temperature lower than100° C. to decompose the dioxins into harmless substances with adecomposition rate of at least 60%.
 2. A process according to claim 1,wherein said aqueous solution contains a contact-accelerating agent. 3.A process according to claim 1, wherein said aqueous solution isirradiated with an ultra sonic wave.
 4. process according to claim 1wherein said catalyst includes a metal ion capable of assuming lowvalency and high valency states.
 5. A process according to claim 1wherein said catalyst is a copper ion or an iron ion.
 6. A processaccording to claim 1 wherein said catalyst contains an undissolvedmatter which is in the course of being converted into a dissolved state.7. A process according to claim 1, wherein said aqueous solution iscontacted with oxygen or an oxygen-containing gas.
 8. A process for thewet processing of a flue gas generated from a combustion furnace andcontaining a dioxin-containing fly ash into a harmless substance,characterized in that said flue gas is contacted with an aqueoussolution, acidified with hydrochloric acid and containing a catalystdissolved therein, at a temperature lower than 100° C. to cause the flyash contained in said flue gas to migrate into said aqueous solution andto decompose the dioxins deposited on the fly ash into a harmlesssubstance with a decomposition rate of at least 60%.
 9. A process forthe wet processing of a flue gas generated from a combustion furnace andcontaining a dioxin-containing fly ash into a harmless substance,characterized in that said process comprises (i) a gas-liquid contactingstep of bringing said flue gas cooled to a temperature lower than 100°C. into gas-liquid contact with an aqueous solution acidified withhydrochloric acid, (ii) a fly ash concentrating step of increasing a flyash content of the aqueous solution obtained in said gas-liquidcontacting step and containing fly ash, and (iii) a dioxin decomposingstep of maintaining said aqueous solution, obtained in said fly ashconcentrating step and containing an increased amount of the fly ash, ata temperature lower than 100° C. in the presence of a catalyst in adissolved state, thereby decomposing the dioxins contained in the flyash into a harmless substance.
 10. A process for the wet processing of aflue gas generated from a combustion furnace and containing adioxin-containing fly ash into a harmless substance, characterized inthat said process comprises (i) a first gas-liquid contacting step ofbringing said flue gas with a first treating liquid, (ii) a secondgas-liquid contacting step of bringing the treated flue gas obtained insaid first gas-liquid contacting step with a second treating liquid, and(iii) a dioxin decomposing step of contacting fly ash “A” captured bysaid first treating liquid in said first gas-liquid contacting step andfly ash “B” captured by said second treating liquid in said secondgas-liquid contacting step, separately or jointly, into contact with anaqueous solution, acidified with hydrochloric acid and containing acatalyst dissolved therein, to decompose the dioxins contained in thefly ash into a harmless substance.
 11. A process according to claim 10,wherein said second gas-liquid contacting step is performed in thepresence of activated carbon.